Theory Coxeter

section ‹Coxeter systems and complexes›

text ‹
  A Coxeter system is a group that affords a presentation, where each generator is of order two,
  and each relator is an alternating word of even length in two generators.

theory Coxeter
imports Chamber


subsection ‹Coxeter-like systems›

text ‹First we work in a group generated by elements of order two.›

subsubsection ‹Locale definition and basic facts›

locale PreCoxeterSystem =
  fixes S :: "'w::group_add set"
  assumes genset_order2: "sS  add_order s = 2"

abbreviation "W  S"
abbreviation "S_length   word_length S"
abbreviation "S_reduced_for  reduced_word_for S"
abbreviation "S_reduced  reduced_word S"
abbreviation "relfun  λs t. add_order (s+t)"

lemma no_zero_genset: "0S"
  assume "0S"
  moreover have "add_order (0::'w) = 1" using add_order0 by fast
  ultimately show False using genset_order2 by simp

lemma genset_order2_add: "sS  s + s = 0"
  using add_order[of s] by (simp add: genset_order2 nataction_2)

lemmas genset_uminus = minus_unique[OF genset_order2_add]

lemma relfun_S: "sS  relfun s s = 1"
  using add_order_relator[of s] by (auto simp add: genset_order2 nataction_2)

lemma relfun_eq1: " sS; relfun s t = 1   t=s"
  using add_order_add_eq1 genset_uminus by fastforce

lemma S_relator_list: "sS  pair_relator_list s s = [s,s]"
  using relfun_S alternating_list2 by simp

lemma S_sym: "T  S  uminus ` T  T"
  using genset_uminus by auto

lemmas special_subgroup_eq_sum_list =
  genby_sym_eq_sum_lists[OF S_sym]
lemmas genby_S_reduced_word_for_arg_min =
  reduced_word_for_genby_sym_arg_min[OF S_sym]
lemmas in_genby_S_reduced_letter_set =
  in_genby_sym_imp_in_reduced_letter_set[OF S_sym]

end (* context PreCoxeterSystem *)

subsubsection ‹Special cosets›

text ‹
  From a Coxeter system we will eventually construct an associated chamber complex. To do so, we
  will consider the collection of special cosets: left cosets of subgroups generated by subsets of
  the generating set @{term S}. This collection forms a poset under the supset relation that, under
  a certain extra assumption, can be used to form a simplicial complex whose poset of simplices
  is isomorphic to this poset of special cosets.  In the literature, groups generated by subsets of
  @{term S} are often referred to as parabolic subgroups of @{term W}, and their cosets as parabolic
  cosets, but following Garrett cite"Garrett:Buildings" we have opted for the names special
  subgroups and special cosets.

context PreCoxeterSystem

definition special_cosets :: "'w set set"
  where "special_cosets  (TPow S. (wW. { w +o T }))"
abbreviation "𝒫  special_cosets"

lemma special_cosetsI: "TPow S  wW  w +o T  𝒫"
  using special_cosets_def by auto

lemma special_coset_singleton: "wW  {w}𝒫"
  using special_cosetsI genby_lcoset_empty[of w] by fastforce

lemma special_coset_nempty: "X𝒫  X  {}"
  using special_cosets_def genby_lcoset_refl by fastforce

lemma special_subgroup_special_coset: "TPow S  T  𝒫"
  using genby_0_closed special_cosetsI[of T] by fastforce

lemma special_cosets_lcoset_closed: "wW  X𝒫  w +o X  𝒫"
  using genby_add_closed unfolding special_cosets_def
  by    (fastforce simp add: set_plus_rearrange2)

lemma special_cosets_lcoset_shift: "wW  ((+o) w) ` 𝒫 = 𝒫"
  using special_cosets_lcoset_closed genby_uminus_closed
  by    (force simp add: set_plus_rearrange2)

lemma special_cosets_has_bottom: "supset_has_bottom 𝒫"
proof (rule ordering.has_bottomI, rule supset_poset)
  show "W𝒫" using special_subgroup_special_coset by fast
  fix X assume X: "X𝒫"
  from this obtain w T where wT: "wW" "TPow S" "X = w +o T"
    using special_cosets_def by auto
  thus "X  W" using genby_mono[of T] genby_lcoset_closed[of w] by auto

lemma special_cosets_bottom: "supset_bottom 𝒫 = W"
proof (rule supset_bottomI[THEN sym])
  fix X assume "X𝒫"
  from this obtain w T where "wW" "TPow S" "X = w +o T"
    using special_cosets_def by auto
  thus "XW"
    using genby_mono[of T S] set_plus_mono[of "T" W] genby_lcoset_el_reduce
    by    force
qed (auto simp add: special_subgroup_special_coset)

end (* context PreCoxeterSystem *)

subsubsection ‹Transfer from the free group over generators›

text ‹
  We form a set of relators and show that it and @{term S} form a
  @{const GroupWithGeneratorsRelators}. The associated quotient group @{term G} maps surjectively
  onto @{term W}. In the CoxeterSystem› locale below, this correspondence will be assumed
  to be injective as well.

context PreCoxeterSystem

abbreviation R :: "'w list set" where "R  (sS. tS. {pair_relator_list s t})"
abbreviation "P  map (charpair S) ` R"
abbreviation "P'  GroupWithGeneratorsRelators.P' S R"
abbreviation "Q  GroupWithGeneratorsRelators.Q S R"
abbreviation "G  GroupWithGeneratorsRelators.G S R"
abbreviation "relator_freeword 
                GroupWithGeneratorsRelators.relator_freeword S"
abbreviation pair_relator_freeword :: "'w  'w  'w freeword"
  where "pair_relator_freeword s t  Abs_freelist (pair_relator_list s t)"

abbreviation "freeliftid  freeword_funlift id"

abbreviation induced_id :: "'w freeword set permutation  'w"
  where "induced_id  GroupWithGeneratorsRelators.induced_id S R"

lemma S_relator_freeword: "sS  pair_relator_freeword s s = s[+]s"
  by (simp add: S_relator_list Abs_freeletter_add)

lemma map_charpair_map_pairtrue_R:
  "sS  tS 
    map (charpair S) (pair_relator_list s t) = map pairtrue (pair_relator_list s t)"
  using set_alternating_list map_charpair_uniform by fastforce

lemma relator_freeword:
  "sS  tS 
    pair_relator_freeword s t = relator_freeword (pair_relator_list s t)"
  using set_alternating_list
        arg_cong[OF map_charpair_map_pairtrue_R, of s t Abs_freeword]
  by    fastforce

lemma relator_freewords: "Abs_freelist ` R = P'"
  using relator_freeword by force

lemma GroupWithGeneratorsRelators_S_R: "GroupWithGeneratorsRelators S R"
  fix rs assume rs: "rsR"
  hence rs': "rs  lists S" using set_alternating_list by fast
  from rs' show "rs  lists (S  uminus ` S)" by fast
  from rs show "sum_list rs = 0" using sum_list_pair_relator_list by fast
  from rs' show "proper_signed_list (map (charpair S) rs)"
    using proper_signed_list_map_uniform_snd
          arg_cong[of "map (charpair S) rs" "map pairtrue rs" proper_signed_list]
    by    fastforce

lemmas GroupByPresentation_S_P =
    OF GroupWithGeneratorsRelators_S_R

lemmas Q_FreeS = GroupByPresentation.Q_FreeS[OF GroupByPresentation_S_P]

lemma relator_freeword_Q: "sS  tS  pair_relator_freeword s t  Q"
  using relator_freeword
        GroupByPresentation.relators[OF GroupByPresentation_S_P]
  by    fastforce

lemmas P'_FreeS =
    OF GroupWithGeneratorsRelators_S_R

lemmas GroupByPresentationInducedFun_S_P_id =
    OF GroupWithGeneratorsRelators_S_R

lemma rconj_relator_freeword:
  " sS; tS; proper_signed_list xs; fst ` set xs  S  
    rconjby (Abs_freeword xs) (pair_relator_freeword s t)  Q"
  using GroupWithGeneratorsRelators.rconj_relator_freeword[
          OF GroupWithGeneratorsRelators_S_R
  by    force

lemma lconjby_Abs_freelist_relator_freeword:
  " sS; tS; xslists S  
    lconjby (Abs_freelist xs) (pair_relator_freeword s t)  Q"
  using GroupWithGeneratorsRelators.lconjby_Abs_freelist_relator_freeword[
          OF GroupWithGeneratorsRelators_S_R
  by    force

lemma Abs_freelist_rev_append_alternating_list_in_Q:
  assumes "sS" "tS"
  shows "Abs_freelist (rev (alternating_list n s t) @ alternating_list n s t)  Q"
proof (induct n)
  case (Suc m)
  define u where "u = (if even m then s else t)"
  define x where "x = Abs_freelist (rev (alternating_list m s t) @ alternating_list m s t)"
  from u_def x_def assms have
    "Abs_freelist (rev (alternating_list (Suc m) s t) @
      alternating_list (Suc m) s t) =
        (pair_relator_freeword u u) + rconjby (Abs_freeletter u) x"
    using Abs_freelist_append[of
            "u # rev (alternating_list m s t) @ alternating_list m s t"
            "rev (alternating_list m s t) @ alternating_list m s t"
    by    (simp add: add.assoc[THEN sym] S_relator_freeword)
  moreover from Suc assms u_def x_def have "rconjby (Abs_freeletter u) x  Q"
    using Abs_freeletter_in_FreeGroup_iff[of _ S]
    by    fastforce
  ultimately show ?case
    using u_def assms relator_freeword_Q genby_add_closed by fastforce
qed (simp add: zero_freeword.abs_eq[THEN sym] genby_0_closed)

lemma Abs_freeword_freelist_uminus_add_in_Q:
  "proper_signed_list xs  fst ` set xs  S 
    - Abs_freelistfst xs + Abs_freeword xs  Q"
proof (induct xs)
  case (Cons x xs)
  from Cons(2) have 1:
    "- Abs_freelistfst (x#xs) + Abs_freeword (x#xs) =
        -Abs_freelistfst xs + -Abs_freeletter (fst x)
          + Abs_freeword [x] + Abs_freeword xs"
    using Abs_freelist_Cons[of "fst x" "map fst xs"]
    by (simp add: Abs_freeword_Cons[THEN sym] add.assoc minus_add)
  show ?case
  proof (cases "snd x")
    case True
    with Cons show ?thesis
      using 1
      by    (simp add:
    case False
    define s where "s = fst x"
    with Cons(3) have s_S: "sS" by simp
    define q where "q = rconjby (Abs_freelistfst xs) (pair_relator_freeword s s)"
    from s_def False Cons(3) have
      "- Abs_freelistfst (x#xs) + Abs_freeword (x#xs) =
        -Abs_freelistfst xs + -pair_relator_freeword s s + Abs_freeword xs"
      using 1 surjective_pairing[of x] S_relator_freeword[of s]
            uminus_Abs_freeword_singleton[of s False, THEN sym]
      by    (simp add: add.assoc)
    with q_def have 2:
      "- Abs_freelistfst (x#xs) + Abs_freeword (x#xs) =
        -q + (-Abs_freelistfst xs + Abs_freeword xs)"
      by (simp add: rconjby_uminus[THEN sym] add.assoc[THEN sym])
    moreover from q_def s_def Cons(3) have "-qQ"
      using proper_signed_list_map_uniform_snd[of True "map fst xs"]
            rconj_relator_freeword genby_uminus_closed
      by    fastforce
    moreover from Cons have "-Abs_freelistfst xs + Abs_freeword xs  Q"
      by (simp add: binrelchain_Cons_reduce)
    ultimately show ?thesis using genby_add_closed by simp
qed (simp add: zero_freeword.abs_eq[THEN sym] genby_0_closed)

lemma Q_freelist_freeword':
  " proper_signed_list xs; fst ` set xs  S; Abs_freelistfst xs  Q  
    Abs_freeword xs  Q"
  using Abs_freeword_freelist_uminus_add_in_Q genby_add_closed
  by    fastforce

lemma Q_freelist_freeword:
  "c  FreeGroup S  Abs_freelist (map fst (freeword c))  Q  c  Q"
 using freeword FreeGroupD Q_freelist_freeword' freeword_inverse[of c]
 by     fastforce

text ‹
  Here we show that the lift of the identity map to the free group on @{term S} is really just

lemma freeliftid_Abs_freeword_conv_sum_list:
  "proper_signed_list xs  fst ` set xs  S 
    freeliftid (Abs_freeword xs) = sum_list (map fst xs)"
  using freeword_funlift_Abs_freeword[of xs id] genset_uminus
        sum_list_map_cong[of xs "apply_sign id" fst]
  by    fastforce

end (* context PreCoxeterSystem *)

subsubsection ‹Words in generators containing alternating subwords›

text ‹
  Besides cancelling subwords equal to relators, the primary algebraic manipulation in seeking to
  reduce a word in generators in a Coxeter system is to reverse the order of alternating subwords
  of half the length of the associated relator, in order to create adjacent repeated letters that
  can be cancelled. Here we detail the mechanics of such manipulations.

context PreCoxeterSystem

lemma sum_list_pair_relator_halflist_flip:
  "sS  tS 
    sum_list (pair_relator_halflist s t) = sum_list (pair_relator_halflist t s)"
  using add_order[of "s+t"] genset_order2_add
        alternating_order2_even_cancel_right[of s t "2*(relfun s t)"]
  by    (simp add: alternating_sum_list_conv_nataction add_order_add_sym)

definition flip_altsublist_adjacent :: "'w list  'w list  bool"
  where "flip_altsublist_adjacent ss ts
           s t as bs. ss = as @ (pair_relator_halflist s t) @ bs 
              ts = as @ (pair_relator_halflist t s) @ bs"
abbreviation "flip_altsublist_chain  binrelchain flip_altsublist_adjacent"

lemma flip_altsublist_adjacentI:
  "ss = as @ (pair_relator_halflist s t) @ bs 
    ts = as @ (pair_relator_halflist t s) @ bs 
    flip_altsublist_adjacent ss ts"
  using flip_altsublist_adjacent_def by fast

lemma flip_altsublist_adjacent_Cons_grow:
  assumes "flip_altsublist_adjacent ss ts"
  shows   "flip_altsublist_adjacent (a#ss) (a#ts)"
  from assms obtain s t as bs
    where ssts: "ss = as @ (pair_relator_halflist s t) @ bs" 
                "ts = as @ (pair_relator_halflist t s) @ bs"
    using flip_altsublist_adjacent_def
    by    auto
  from ssts have
    "a#ss = (a#as) @ (pair_relator_halflist s t) @ bs"
    "a#ts = (a#as) @ (pair_relator_halflist t s) @ bs"
    by auto
  thus ?thesis by (fast intro: flip_altsublist_adjacentI)

lemma flip_altsublist_chain_map_Cons_grow:
  "flip_altsublist_chain tss  flip_altsublist_chain (map ((#) t) tss)"
  by  (induct tss rule: list_induct_CCons)
      (auto simp add:
        binrelchain_Cons_reduce[of flip_altsublist_adjacent]

lemma flip_altsublist_adjacent_refl:
  "ss  []  sslists S  flip_altsublist_adjacent ss ss"
proof (induct ss rule: list_nonempty_induct)
  case (single s)
  hence "[s] = [] @ pair_relator_halflist s s @ []"
    using relfun_S by simp
  thus ?case by (fast intro: flip_altsublist_adjacentI)
  case cons thus ?case using flip_altsublist_adjacent_Cons_grow by simp

lemma flip_altsublist_adjacent_sym:
  "flip_altsublist_adjacent ss ts  flip_altsublist_adjacent ts ss"
  using flip_altsublist_adjacent_def flip_altsublist_adjacentI by auto

lemma rev_flip_altsublist_chain:
  "flip_altsublist_chain xss  flip_altsublist_chain (rev xss)"
  using flip_altsublist_adjacent_sym binrelchain_snoc[of flip_altsublist_adjacent]
  by    (induct xss rule: list_induct_CCons) auto

lemma flip_altsublist_adjacent_set:
  assumes "sslists S" "flip_altsublist_adjacent ss ts"
  shows   "set ts = set ss"
  from assms obtain s t as bs where ssts:  
    "ss = as @ (pair_relator_halflist s t) @ bs"
    "ts = as @ (pair_relator_halflist t s) @ bs"
    using flip_altsublist_adjacent_def
    by    auto
  with assms(1) show ?thesis
    using set_alternating_list2[of "relfun s t" s t]
          set_alternating_list2[of "relfun t s" t s]
          add_order_add_sym[of t s] relfun_eq1
    by    (cases "relfun s t" rule: nat_cases_2Suc) auto

lemma flip_altsublist_adjacent_set_ball:
  "sslists S. ts. flip_altsublist_adjacent ss ts  set ts = set ss"
  using flip_altsublist_adjacent_set by fast

lemma flip_altsublist_adjacent_lists:
  "ss  lists S  flip_altsublist_adjacent ss ts  ts  lists S"
  using flip_altsublist_adjacent_set by fast

lemma flip_altsublist_adjacent_lists_ball:
  "sslists S. ts. flip_altsublist_adjacent ss ts  ts  lists S"
  using flip_altsublist_adjacent_lists by fast

lemma flip_altsublist_chain_lists:
  "ss  lists S  flip_altsublist_chain (ss#xss@[ts])  ts  lists S"
  using flip_altsublist_adjacent_lists
          "λss. sslists S" flip_altsublist_adjacent ss xss ts
  by    fast

lemmas flip_altsublist_chain_funcong_Cons_snoc =
  binrelchain_setfuncong_Cons_snoc[OF flip_altsublist_adjacent_lists_ball]

lemmas flip_altsublist_chain_set =
    OF flip_altsublist_adjacent_set_ball

lemma flip_altsublist_adjacent_length:
  "flip_altsublist_adjacent ss ts  length ts = length ss"
  unfolding flip_altsublist_adjacent_def
  by        (auto simp add: add_order_add_sym length_alternating_list)

lemmas flip_altsublist_chain_length =
    of flip_altsublist_adjacent length, OF flip_altsublist_adjacent_length, simplified

lemma flip_altsublist_adjacent_sum_list:
  assumes "ss  lists S" "flip_altsublist_adjacent ss ts"
  shows "sum_list ts = sum_list ss"
  from assms(2) obtain s t as bs where stasbs:
    "ss = as @ (pair_relator_halflist s t) @ bs"
    "ts = as @ (pair_relator_halflist t s) @ bs"
    using flip_altsublist_adjacent_def
    by    auto
  show ?thesis
  proof (cases "relfun s t")
    case 0 thus ?thesis using stasbs by (simp add: add_order_add_sym)
    case Suc
    with assms stasbs have "sS" "tS"
      using set_alternating_list1[of "add_order (s+t)" s t]
            set_alternating_list1[of "add_order (t+s)" t s]
            add_order_add_sym[of t]
            flip_altsublist_adjacent_lists[of ss ts]
      by    auto
    with stasbs show ?thesis
      using sum_list_pair_relator_halflist_flip by simp

lemma flip_altsublist_adjacent_sum_list_ball:
  "sslists S. ts. flip_altsublist_adjacent ss ts  sum_list ts = sum_list ss"
  using flip_altsublist_adjacent_sum_list by fast

lemma S_reduced_forI_flip_altsublist_adjacent:
  "S_reduced_for w ss  flip_altsublist_adjacent ss ts  S_reduced_for w ts"
  using reduced_word_for_lists[of S] reduced_word_for_sum_list
        flip_altsublist_adjacent_lists flip_altsublist_adjacent_sum_list
  by    (fastforce intro: reduced_word_forI_compare)

lemma flip_altsublist_adjacent_in_Q':
  fixes   as bs s t
  defines xs: "xs  as @ pair_relator_halflist s t @ bs"
  and     ys: "ys  as @ pair_relator_halflist t s @ bs"
  assumes Axs: "Abs_freelist xs  Q"
  shows "Abs_freelist ys  Q"
  define X Y A B half_st half2_st half_ts
    where "X = Abs_freelist xs"
      and "Y = Abs_freelist ys"
      and "A = Abs_freelist as"
      and "B = Abs_freelist bs"
      and "half_st = Abs_freelist (pair_relator_halflist s t)"
      and "half2_st = Abs_freelist (pair_relator_halflist2 s t)"
      and "half_ts = Abs_freelist (pair_relator_halflist t s)"
  define z where "z = -half2_st + B"
  define w1 w2 where "w1 = rconjby z (pair_relator_freeword s t)"
    and "w2 = Abs_freelist (rev (pair_relator_halflist t s) @ pair_relator_halflist t s)"
  define w3 where "w3 = rconjby B w2"

  from w1_def z_def
    have  w1': "w1 = rconjby B (lconjby half2_st (pair_relator_freeword s t))"
    by    (simp add: rconjby_add)
  hence "-w1 = rconjby B (lconjby half2_st (-pair_relator_freeword s t))"
    using lconjby_uminus[of "half2_st"] by (simp add: rconjby_uminus[THEN sym])
  moreover from X_def xs A_def half_st_def B_def have "X = A + B + rconjby B half_st"
    by  (simp add:
          Abs_freelist_append_append[THEN sym] add.assoc[THEN sym]
  ultimately have
    "X + -w1 = A + B +
      ( rconjby B (half_st + (half2_st + -pair_relator_freeword s t - half2_st)) )"
    by (simp add: add.assoc add_rconjby)
  moreover from w2_def half2_st_def half_ts_def have "w2 = half2_st + half_ts"
    by  (simp add:
          Abs_freelist_append[THEN sym]
  ultimately have
    "X + -w1 + w3 = A + B + (rconjby B (-half2_st + (half2_st + half_ts)))"
    using half_st_def half2_st_def w3_def add_assoc4[
            of half_st half2_st "-pair_relator_freeword s t" "-half2_st"
    by    (simp add:
            Abs_freelist_append[THEN sym] pair_relator_halflist_append
            add.assoc add_rconjby
  hence Y': "Y = X - w1 + w3"
    using A_def half_ts_def B_def ys Y_def
    by    (simp add:
            add.assoc[THEN sym]
            Abs_freelist_append_append[THEN sym]

  from Axs have xs_S: "xs  lists S" using Q_FreeS FreeGroupD_transfer' by fast
  have "w1Q  w3Q"
  proof (cases "relfun s t")
    case 0 with w1_def w2_def w3_def show ?thesis using genby_0_closed
      by  (auto simp add:
            zero_freeword.abs_eq[THEN sym]
    case (Suc m) have m: "add_order (s+t) = Suc m" by fact
    have st: "{s,t}  S"
    proof (cases m)
      case 0 with m xs xs_S show ?thesis
        using set_alternating_list1 relfun_eq1 by force
      case Suc with m xs xs_S show ?thesis
        using set_alternating_list2[of "add_order (s+t)" s t] by fastforce
    from xs xs_S B_def have B_S: "B  FreeGroup S"
      using Abs_freelist_in_FreeGroup[of bs S] by simp
    moreover from w2_def have "w2Q"
      using st Abs_freelist_rev_append_alternating_list_in_Q[of t s "add_order (t+s)"]
      by    fast
    ultimately have "w3  Q"
      using w3_def FreeGroup_genby_set_lconjby_set_rconjby_closed by fast
    moreover from half2_st_def have "w1  Q"
      using w1' st B_S alternating_list_in_lists[of s S] alternating_list_in_lists[of t S]
            lconjby_Abs_freelist_relator_freeword[of s t]
      by    (force intro: FreeGroup_genby_set_lconjby_set_rconjby_closed)
    ultimately show ?thesis by fast
  with X_def Y_def Axs show ?thesis
    using Y' genby_diff_closed[of X] genby_add_closed[of "X-w1" _ w3] by simp


lemma flip_altsublist_adjacent_in_Q:
  "Abs_freelist ss  Q  flip_altsublist_adjacent ss ts  Abs_freelist ts  Q"
  using flip_altsublist_adjacent_def flip_altsublist_adjacent_in_Q' by auto

lemma flip_altsublist_chain_G_in_Q:
  " Abs_freelist ss  Q; flip_altsublist_chain (ss#xss@[ts])   Abs_freelist ts  Q"
  using flip_altsublist_adjacent_in_Q
          "λss. Abs_freelist ss  Q"
  by    fast

lemma alternating_S_no_flip:
  assumes "sS" "tS" "n > 0" "n < relfun s t  relfun s t = 0"
  shows   "sum_list (alternating_list n s t)  sum_list (alternating_list n t s)"
  assume "sum_list (alternating_list n s t) = sum_list (alternating_list n t s)"
  hence "sum_list (alternating_list n s t) + - sum_list (alternating_list n t s) = 0"
    by simp
  with assms(1,2) have "sum_list (alternating_list (2*n) s t) = 0"
    by  (cases "even n")
        (auto simp add:
          genset_order2_add uminus_sum_list_alternating_order2
          sum_list.append[THEN sym]
          alternating_list_append mult_2
  with assms(3,4) less_add_order_eq_0_contra add_order_eq0 show False
    by (auto simp add: alternating_sum_list_conv_nataction)

lemma exchange_alternating_not_in_alternating:
  assumes "n  2" "n < relfun s t  relfun s t = 0"
          "S_reduced_for w (alternating_list n s t @ cs)"
          "alternating_list n s t @ cs = xs@[x]@ys" "S_reduced_for w (t#xs@ys)"
  shows   "length xs  n"
  from assms(1) obtain m k where n: "n = Suc m" and m: "m = Suc k"
    using gr0_implies_Suc by fastforce
  define altnst altnts altmts altkst
    where "altnst = alternating_list n s t"
    and "altnts = alternating_list n t s"
    and "altmts = alternating_list m t s"
    and "altkst = alternating_list k s t"
  from altnst_def altmts_def n have altnmst: "altnst = s # altmts"
    using alternating_list_Suc_Cons[of m] by fastforce
  with assms(3) altnst_def have s_S: "sS" using reduced_word_for_lists by fastforce
  from assms(5) have t_S: "tS" using reduced_word_for_lists by fastforce
  from m altnmst altmts_def altkst_def have altnkst: "altnst = s # t # altkst"
    using alternating_list_Suc_Cons by fastforce
  have "¬ length xs < n"
  proof (cases "Suc (length xs) = n")
    case True
    with assms(4,5) n altnts_def have flip: "S_reduced_for w (altnts @ cs)"
      using length_alternating_list[of n s t]
            alternating_list_Suc_Cons[of m t s]
      by    auto
    from altnst_def have "sum_list altnst = sum_list altnts"
      using reduced_word_for_sum_list[OF assms(3)]
            reduced_word_for_sum_list[OF flip]
      by    auto
    with n assms(2) altnst_def altnts_def show ?thesis
      using alternating_S_no_flip[OF s_S t_S] by fast
    case False show ?thesis
    proof (cases xs ys rule: two_lists_cases_snoc_Cons)
      case Nil1
      from Nil1(1) assms(4) altnkst altnst_def have "ys = t # altkst @ cs" by auto
      with Nil1(1) assms(5) show ?thesis
        using t_S genset_order2_add[of t]
              contains_order2_nreduced[of t S "[]" "altkst@cs"]
        by    force
      case Nil2 with assms(4) altnst_def False show ?thesis
        using length_append[of altnst cs]
        by    (fastforce simp add: length_alternating_list)
      case (snoc_Cons us u z zs)
      with assms(4,5) altnst_def
        have  1: "altnst @ cs = us@[u,x,z]@zs" "S_reduced_for w (t#us@[u,z]@zs)"
        by    auto
      from 1(1) snoc_Cons(1) False altnst_def show ?thesis
        using take_append[of n altnst cs] take_append[of n "us@[u,x,z]" zs]
              set_alternating_list[of n s t]
              alternating_list_alternates[of n s t us u]
              reduced_word_for_imp_reduced_word[OF 1(2)]
              s_S t_S genset_order2_add 
              contains_order2_nreduced[of u S "t#us"]
        by    (force simp add: length_alternating_list)
  thus ?thesis by fastforce

end (* context PreCoxeterSystem *)

subsubsection ‹Preliminary facts on the word problem›

text ‹
  The word problem seeks criteria for determining whether two words over the generator set represent
  the same element in @{term W}. Here we establish one direction of the word problem, as well as a
  preliminary step toward the other direction.

context PreCoxeterSystem

lemmas flip_altsublist_chain_sum_list =
  flip_altsublist_chain_funcong_Cons_snoc[OF flip_altsublist_adjacent_sum_list_ball]
― ‹This lemma represents one direction in the word problem: if a word in generators can be
transformed into another by a sequence of manipulations, each of which consists of replacing a
half-relator subword by its reversal, then the two words sum to the same element of @{term W}.›

lemma reduced_word_problem_eq_hd_step:
  assumes step: "y ss ts. 
                  S_length y < S_length w; y0; S_reduced_for y ss; S_reduced_for y ts
                  xss. flip_altsublist_chain (ss # xss @ [ts])"
  and     set_up: "S_reduced_for w (a#ss)" "S_reduced_for w (a#ts)"
  shows   "xss. flip_altsublist_chain ((a#ss) # xss @ [a#ts])"
proof (cases "ss=ts")
  case True
  with set_up(1) have "flip_altsublist_chain ((a#ss) # [] @ [a#ts])"
    using reduced_word_for_lists flip_altsublist_adjacent_refl by fastforce
  thus ?thesis by fast
  case False
  define y where "y = sum_list ss"
  with set_up(1) have ss: "S_reduced_for y ss"
    using reduced_word_for_imp_reduced_word reduced_word_Cons_reduce by fast
  moreover from y_def ss have ts: "S_reduced_for y ts"
    using reduced_word_for_sum_list[OF set_up(1)]
          reduced_word_for_sum_list[OF set_up(2)]
          reduced_word_for_eq_length[OF set_up(1) set_up(2)]
          reduced_word_for_lists[OF set_up(2)]
    by    (auto intro: reduced_word_forI_compare)
  moreover from ss set_up(1) have "S_length y < S_length w"
    using reduced_word_for_length reduced_word_for_length by fastforce
  moreover from False have "y  0"
    using ss ts reduced_word_for_0_imp_nil reduced_word_for_0_imp_nil by fastforce
  ultimately show ?thesis
    using step flip_altsublist_chain_map_Cons_grow by fastforce

end (* context PreCoxeterSystem *)

subsubsection ‹Preliminary facts related to the deletion condition›

text ‹
  The deletion condition states that in a Coxeter system, every non-reduced word in the generating
  set can be shortened to an equivalent word by deleting some particular pair of letters. This
  condition is both necessary and sufficient for a group generated by elements of order two to be a
  Coxeter system. Here we establish some facts related to the deletion condition that are true in
  any group generated by elements of order two.

context PreCoxeterSystem

abbreviation "  (wW. lconjby w ` S)" ― ‹the set of reflections›

abbreviation "lift_signed_lconjperm  freeword_funlift signed_lconjpermutation"

lemma lconjseq_reflections: "sslists S  set (lconjseq ss)  "
  using special_subgroup_eq_sum_list[of S]
  by    (induct ss rule: rev_induct) (auto simp add: lconjseq_snoc)

lemma deletion':
  "ss  lists S  ¬ distinct (lconjseq ss) 
    a b as bs cs. ss = as @ [a] @ bs @ [b] @ cs 
      sum_list ss = sum_list (as@bs@cs)"
proof (induct ss)
  case (Cons s ss)
  show ?case
  proof (cases "distinct (lconjseq ss)")
    case True with Cons(2,3) show ?thesis
      using subset_inj_on[OF lconjby_inj, of "set (lconjseq ss)" s]
            distinct_map[of "lconjby s"]
            genset_order2_add order2_hd_in_lconjseq_deletion[of s ss]
      by    (force simp add: algebra_simps)
    case False
    with Cons(1,2) obtain a b as bs cs where
      "s#ss = (s#as) @ [a] @ bs @ [b] @ cs"
      "sum_list (s#ss) = sum_list ((s#as) @ bs @ cs)"
      by    auto
    thus ?thesis by fast
qed simp

lemma S_reduced_imp_distinct_lconjseq':
  assumes "ss  lists S" "¬ distinct (lconjseq ss)"
  shows "¬ S_reduced ss"
  assume ss: "S_reduced ss"
  from assms obtain as a bs b cs
    where decomp: "ss = as @ [a] @ bs @ [b] @ cs"
                  "sum_list ss = sum_list (as@bs@cs)"
    using deletion'[of ss]
    by    fast
  from ss decomp assms(1) show False
    using reduced_word_for_minimal[of S _ ss "as@bs@cs"] by auto

lemma S_reduced_imp_distinct_lconjseq: "S_reduced ss  distinct (lconjseq ss)"
  using reduced_word_for_lists S_reduced_imp_distinct_lconjseq' by fast  

lemma permutation_lift_signed_lconjperm_eq_signed_list_lconjaction':
  "proper_signed_list xs  fst ` set xs  S 
    permutation (lift_signed_lconjperm (Abs_freeword xs)) =
      signed_list_lconjaction (map fst xs)"
proof (induct xs)
  case Nil
  have "Abs_freeword ([]::'w signed list) = (0::'w freeword)"
    using zero_freeword.abs_eq by simp
  thus ?case by (simp add: zero_permutation.rep_eq freeword_funlift_0)
  case (Cons x xs)
  obtain s b where x: "x=(s,b)" by fastforce
  with Cons show ?case
    using Abs_freeword_Cons[of x xs]
          binrelchain_Cons_reduce[of nflipped_signed x xs]
          bij_signed_lconjaction[of s] genset_order2_add[of s]
    by    (cases b)
          (auto simp add:
            plus_permutation.rep_eq freeword_funlift_add
            Abs_permutation_inverse uminus_permutation.rep_eq

lemma permutation_lift_signed_lconjperm_eq_signed_list_lconjaction:
  "x  FreeGroup S 
    permutation (lift_signed_lconjperm x) =
      signed_list_lconjaction (map fst (freeword x))"
  using freeword FreeGroup_def[of S] freeword_inverse[of x]
  by    force

lemma even_count_lconjseq_rev_relator:
  "sS  tS  even (count_list (lconjseq (rev (pair_relator_list s t))) x)"
  using even_count_lconjseq_alternating_order2[of t]
  by    (simp add: genset_order2_add add_order rev_pair_relator_list)

lemma GroupByPresentationInducedFun_S_R_signed_lconjaction:
  "GroupByPresentationInducedFun S P signed_lconjpermutation"
proof (intro_locales, rule GroupByPresentation_S_P, unfold_locales)
  fix ps assume ps: "psP"
  define r where "r = Abs_freeword ps"
  with ps have r: "rP'" by fast
  then obtain s t where st: "sS" "tS" "r = pair_relator_freeword s t"
    using relator_freewords by fast
  from r st(3)
    have 1: "permutation (lift_signed_lconjperm r) =
              signed_list_lconjaction (pair_relator_list s t)"
    using P'_FreeS
          Abs_freelist_inverse[of "pair_relator_list s t"]
          map_fst_map_const_snd[of True "pair_relator_list s t"]
    by    force
  have "permutation (lift_signed_lconjperm r) = id"
    fix x
    show "lift_signed_lconjperm r  x = id x"
      show "snd (freeword_funlift signed_lconjpermutation r  x) = snd (id x)"
        using 1 st(1,2) even_count_lconjseq_rev_relator genset_order2_add
              set_alternating_list[of "2*relfun s t" s t]
              signed_list_lconjaction_snd[of "pair_relator_list s t" x]
        by    fastforce
    qed (simp add: 1 signed_list_lconjaction_fst sum_list_pair_relator_list)
    have  "permutation (0::'w signed permutation) = (id::'w signed  'w signed)"
    using zero_permutation.rep_eq
    by    fast
  ultimately show "lift_signed_lconjperm r = 0"
    using permutation_inject by fastforce

end (* context PreCoxeterSystem *)

subsection ‹Coxeter-like systems with deletion›

text ‹
  Here we add the so-called deletion condition as an assumption, and explore its consequences.

subsubsection ‹Locale definition›

locale PreCoxeterSystemWithDeletion = PreCoxeterSystem S
  for S :: "'w::group_add set"
+ assumes deletion:
  "ss  lists S  ¬ reduced_word S ss 
    a b as bs cs. ss = as @ [a] @ bs @ [b] @ cs 
      sum_list ss = sum_list (as@bs@cs)"

subsubsection ‹Consequences of the deletion condition›

context PreCoxeterSystemWithDeletion

lemma deletion_reduce:
  "ss  lists S  ts. ts  ssubseqs ss  reduced_words_for S (sum_list ss)"
proof (cases "S_reduced ss")
  case True
  thus  "ss  lists S 
          ts. ts  ssubseqs ss  reduced_words_for S (sum_list ss)"
    by  (force simp add: ssubseqs_refl)
  case False
  have "ss  lists S  ¬ S_reduced ss 
        ts. ts  ssubseqs ss  reduced_words_for S (sum_list ss)"
  proof (induct ss rule: length_induct)
    fix xs::"'w list"
    assume xs:
      "ys. length ys < length xs  ys  lists S  ¬ S_reduced ys
         (ts. ts  ssubseqs ys  reduced_words_for S (sum_list ys))"
      "xs  lists S" "¬ S_reduced xs"
    from xs(2,3) obtain as a bs b cs
      where asbscs: "xs = as@[a]@bs@[b]@cs" "sum_list xs = sum_list (as@bs@cs)"
      using deletion[of xs]
      by    fast
    show "ts. ts  ssubseqs xs  reduced_words_for S (sum_list xs)"
    proof (cases "S_reduced (as@bs@cs)")
      case True with asbscs xs(2) show ?thesis
        using delete2_ssubseqs by fastforce
      case False
      moreover from asbscs(1) xs(2)
        have  "length (as@bs@cs) < length xs" "as@bs@cs  lists S"
        by    auto
      ultimately obtain ts
        where ts: "ts  ssubseqs (as@bs@cs) 
                    reduced_words_for S (sum_list (as@bs@cs))"
        using xs(1,2) asbscs(1)
        by    fast
      with asbscs show ?thesis
        using delete2_ssubseqs[of as bs cs a b] ssubseqs_subset by auto
  with False
    show  "ss  lists S 
            ts. ts  ssubseqs ss  reduced_words_for S (sum_list ss)"
    by    fast

lemma deletion_reduce':
  "ss  lists S  tsreduced_words_for S (sum_list ss). set ts  set ss"
  using deletion_reduce[of ss] subseqs_powset[of ss] by auto

end (* context PreCoxeterSystemWithDeletion *)

subsubsection ‹The exchange condition›

text ‹
  The exchange condition states that, given a reduced word in the generators, if prepending a
  letter to the word does not remain reduced, then the new word can be shortened to a word
  equivalent to the original one by deleting some letter other than the prepended one. Thus, one
  able to exchange some letter for the addition of a desired letter at the beginning of a word,
  without changing the elemented represented.

context PreCoxeterSystemWithDeletion

lemma exchange:
  assumes "sS" "S_reduced_for w ss" "¬ S_reduced (s#ss)"
  shows   "t as bs. ss = as@t#bs  reduced_word_for S w (s#as@bs)"
  from assms(2) have ss_lists: "ss  lists S" using reduced_word_for_lists by fast
  with assms(1) have "s#ss  lists S" by simp
  with assms(3) obtain a b as bs cs
    where del:  "s#ss = as @ [a] @ bs @ [b] @ cs"
                "sum_list (s#ss) = sum_list (as@bs@cs)"
    using deletion[of "s#ss"]
    by    fastforce
  show ?thesis
  proof (cases as)
    case Nil with assms(1,2) del show ?thesis
      using reduced_word_for_sum_list add.assoc[of s s w] genset_order2_add ss_lists
      by    (fastforce intro: reduced_word_forI_compare)
    case (Cons d ds) with del assms(2) show ?thesis
      using ss_lists reduced_word_for_imp_reduced_word
            reduced_word_for_minimal[of S "sum_list ss" ss "ds@bs@cs"]
      by    fastforce

lemma reduced_head_imp_exchange:
  assumes "reduced_word_for S w (s#as)" "reduced_word_for S w cs"
  shows   "a ds es. cs = ds@[a]@es  reduced_word_for S w (s#ds@es)"
  from assms(1) have s_S: "sS" using reduced_word_for_lists by fastforce
  moreover have "¬ S_reduced (s#cs)"
  proof (rule not_reduced_word_for)
    show "as  lists S" using reduced_word_for_lists[OF assms(1)] by simp
    from assms(1,2) show "sum_list as = sum_list (s#cs)"
      using s_S reduced_word_for_sum_list[of S w] add.assoc[of s s] genset_order2_add
      by    fastforce
    from assms(1,2) show "length as < length (s#cs)"
      using reduced_word_for_length[of S w] by fastforce
  ultimately obtain a ds es
    where "cs = ds@[a]@es" "reduced_word_for S w (s#ds@es)"
    using assms(2) exchange[of s w cs]
    by    auto
  thus ?thesis by fast

end (* context PreCoxeterSystemWithDeletion *)

subsubsection ‹More on words in generators containing alternating subwords›

text ‹
  Here we explore more of the mechanics of manipulating words over @{term S} that contain
  alternating subwords, in preparation of the word problem.

context PreCoxeterSystemWithDeletion

lemma two_reduced_heads_imp_reduced_alt_step:
  assumes "st" "reduced_word_for S w (t#bs)" "n < relfun s t  relfun s t = 0"
          "reduced_word_for S w (alternating_list n s t @ cs)"
  shows   "ds. reduced_word_for S w (alternating_list (Suc n) t s @ ds)"
  define altnst where "altnst = alternating_list n s t"
  with assms(2,4) obtain x xs ys 
    where xxsys: "altnst @ cs = xs@[x]@ys" "reduced_word_for S w (t#xs@ys)"
    using reduced_head_imp_exchange
    by    fast
  show ?thesis
  proof (cases n rule: nat_cases_2Suc)
    case 0 with xxsys(2) show ?thesis by auto
    case 1 with assms(1,4) xxsys altnst_def show ?thesis
      using reduced_word_for_sum_list[of S w "s#cs"]
            reduced_word_for_sum_list[of S w "t#cs"]
      by    (cases xs) auto
    case (SucSuc k)
    with assms(3,4) xxsys altnst_def have "length xs  n"
      using exchange_alternating_not_in_alternating by simp
    moreover define ds where "ds = take (length xs - n) cs"
    ultimately have "t#xs@ys = alternating_list (Suc n) t s @ ds @ ys"
      using xxsys(1) altnst_def take_append[of "length xs" altnst cs]
            alternating_list_Suc_Cons[of n t]
      by    (fastforce simp add: length_alternating_list)
    with xxsys(2) show ?thesis by auto

lemma two_reduced_heads_imp_reduced_alt':
  assumes "st" "reduced_word_for S w (s#as)" "reduced_word_for S w (t#bs)"
  shows "n  relfun s t  relfun s t = 0  (cs.
          reduced_word_for S w (alternating_list n s t @ cs) 
          reduced_word_for S w (alternating_list n t s @ cs)
proof (induct n)
  case 0 from assms(2) show ?case by auto
  case (Suc m) thus ?case
    using add_order_add_sym[of s t]
            OF assms(1)[THEN not_sym] assms(2), of m
          two_reduced_heads_imp_reduced_alt_step[OF assms(1,3), of m]
    by    fastforce

lemma two_reduced_heads_imp_reduced_alt:
  assumes "st" "reduced_word_for S w (s#as)" "reduced_word_for S w (t#bs)"
  shows "cs. reduced_word_for S w (pair_relator_halflist s t @ cs)"
  define altst altts
    where "altst = pair_relator_halflist s t"
      and "altts = pair_relator_halflist t s"
  then obtain cs
    where cs: "reduced_word_for S w (altst @ cs) 
                reduced_word_for S w (altts @ cs)"
    using add_order_add_sym[of t] two_reduced_heads_imp_reduced_alt'[OF assms]
    by    auto
  moreover from altst_def altts_def
    have  "reduced_word_for S w (altts @ cs)  reduced_word_for S w (altst @ cs)"
    using reduced_word_for_lists[OF assms(2)] reduced_word_for_lists[OF assms(3)]
    by (force intro: S_reduced_forI_flip_altsublist_adjacent
        simp add: add_order_add_sym)
  ultimately show "cs. reduced_word_for S w (altst @ cs)" by fast

lemma two_reduced_heads_imp_nzero_relfun:
  assumes "st" "reduced_word_for S w (s#as)" "reduced_word_for S w (t#bs)"
  shows   "relfun s t  0"
  assume 1: "relfun s t = 0"
  define altst altts
    where "altst = alternating_list (Suc (S_length w)) s t"
      and "altts = alternating_list (Suc (S_length w)) t s"
  with 1 obtain cs
    where "reduced_word_for S w (altst @ cs) 
            reduced_word_for S w (altts @ cs)"
    using two_reduced_heads_imp_reduced_alt'[OF assms]
    by    fast
  moreover from altst_def altts_def
    have  "length (altst @ cs) > S_length w"
          "length (altts @ cs) > S_length w"
    using length_alternating_list[of _ s] length_alternating_list[of _ t]
    by    auto
  ultimately show False using reduced_word_for_length by fastforce

end (* context PreCoxeterSystemWithDeletion *)

subsubsection ‹The word problem›

text ‹Here we establish the other direction of the word problem for reduced words.›

context PreCoxeterSystemWithDeletion

lemma reduced_word_problem_ConsCons_step:
  assumes "y ss ts.  S_length y < S_length w; y0; reduced_word_for S y ss;
            reduced_word_for S y ts   xss. flip_altsublist_chain (ss # xss @ [ts])"
          "reduced_word_for S w (a#as)" "reduced_word_for S w (b#bs)" "ab"
  shows   "xss. flip_altsublist_chain ((a#as)#xss@[b#bs])"
  from assms(2-4) obtain cs
    where cs: "reduced_word_for S w (pair_relator_halflist a b @ cs)"
    using two_reduced_heads_imp_reduced_alt
    by    fast
  define rs us where "rs = pair_relator_halflist a b @ cs"
    and "us = pair_relator_halflist b a @ cs"
  from assms(2,3) have a_S: "aS" and b_S: "bS" 
    using reduced_word_for_lists[of S _ "a#as"] reduced_word_for_lists[of S _ "b#bs"]
    by    auto
  with rs_def us_def have midlink: "flip_altsublist_adjacent rs us"
    using add_order_add_sym[of b a] flip_altsublist_adjacent_def by fastforce
  from assms(2-4) have "relfun a b  0"
    using two_reduced_heads_imp_nzero_relfun by fast
  from this obtain k where k: "relfun a b = Suc k"
    using not0_implies_Suc by auto
  define qs vs
    where "qs = alternating_list k b a @ cs"
      and "vs = alternating_list k a b @ cs"
  with k rs_def us_def have rs': "rs = a # qs" and us': "us = b # vs"
    using add_order_add_sym[of b a] alternating_list_Suc_Cons[of k] by auto
  from assms(1,2) cs rs_def rs'
    have  startlink: "as  qs  xss. flip_altsublist_chain ((a#as) # xss @ [rs])"
    using reduced_word_problem_eq_hd_step
    by    fastforce
  from assms(1,3) rs_def cs us'
    have  endlink: "bs  vs  xss. flip_altsublist_chain (us # xss @ [b#bs])"
    using midlink flip_altsublist_adjacent_sym
          S_reduced_forI_flip_altsublist_adjacent[of w rs]
          reduced_word_problem_eq_hd_step[of w]
    by    auto
  show ?thesis
  proof (cases "as = qs" "bs = vs" rule: two_cases)
    case both
    with rs' us' have "flip_altsublist_chain ((a#as) # [] @ [b#bs])"
      using midlink by simp
    thus ?thesis by fast
    case one
    with rs' obtain xss
      where "flip_altsublist_chain ((a#as) # (us # xss) @ [b#bs])" 
      using endlink midlink
      by    auto
    thus ?thesis by fast
    case other
    from other(1) obtain xss where "flip_altsublist_chain ((a#as) # xss @ [rs])"
      using startlink by fast
    with other(2) us' startlink
      have  "flip_altsublist_chain ((a#as) # (xss@[rs]) @ [b#bs])"
      using midlink binrelchain_snoc[of flip_altsublist_adjacent "(a#as)#xss"]
      by    simp
    thus ?thesis by fast
    case neither
    from neither(1) obtain xss
      where "flip_altsublist_chain ((a#as) # xss @ [rs])"
      using startlink
      by    fast
    with neither(2) obtain yss
      where "flip_altsublist_chain ((a#as) # (xss @ [rs,us] @ yss) @ [b#bs])"
      using startlink midlink endlink
            binrelchain_join[of flip_altsublist_adjacent "(a#as)#xss"]
      by    auto
    thus ?thesis by fast

lemma reduced_word_problem:
  " w0; reduced_word_for S w ss; reduced_word_for S w ts  
    xss. flip_altsublist_chain (ss#xss@[ts])"
proof (induct w arbitrary: ss ts rule: measure_induct_rule[of "S_length"])
  case (less w)
  show ?case
  proof (cases ss ts rule: two_lists_cases_Cons_Cons)
    case Nil1 from Nil1(1) less(2,3) show ?thesis
      using reduced_word_for_sum_list by fastforce
    case Nil2 from Nil2(2) less(2,4) show ?thesis
      using reduced_word_for_sum_list by fastforce
    case (ConsCons a as b bs)
    show ?thesis
    proof (cases "a=b")
      case True with less ConsCons show ?thesis
        using reduced_word_problem_eq_hd_step[of w] by auto
      case False with less ConsCons show ?thesis
        using reduced_word_problem_ConsCons_step[of w] by simp

lemma reduced_word_letter_set:
  assumes "S_reduced_for w ss"
  shows "reduced_letter_set S w = set ss"
proof (cases "w=0")
  case True with assms show ?thesis
    using reduced_word_for_0_imp_nil[of S ss] reduced_letter_set_0 by simp
  case False
  show ?thesis
    from assms show "set ss  reduced_letter_set S w" by fast
    show "reduced_letter_set S w  set ss"
      fix x assume "x  reduced_letter_set S w"
      from this obtain ts where "reduced_word_for S w ts" "x  set ts" by fast
      with False assms show "x  set ss"
        using reduced_word_for_lists[of S _ ss] reduced_word_problem[of w ss]
        by    force

end (* context PreCoxeterSystemWithDeletion *)

subsubsection ‹Special subgroups and cosets›

text ‹
  Recall that special subgroups are those generated by subsets of the generating set @{term S}.
  Here we show that the presence of the deletion condition guarantees that the collection of
  special subgroups and their left cosets forms a poset under reverse inclusion that satisfies the
  necessary properties to ensure that the poset of simplices in the associated simplicial complex
  is isomorphic to this poset of special cosets.

context PreCoxeterSystemWithDeletion

lemma special_subgroup_int_S:
  assumes "T  Pow S"
  shows   "T  S = T"
  show "T  S  T"
    fix t assume t: "t  T  S"
    with assms obtain ts where ts: "ts  lists T" "t = sum_list ts"
      using special_subgroup_eq_sum_list[of T] by fast
    with assms obtain us
      where us: "reduced_word_for S (sum_list ts) us" "set us  set ts"
      using deletion_reduce'[of ts]
      by    auto
    with no_zero_genset ts(2) t have "length us = 1"
      using reduced_word_for_lists[of S _ us] reduced_word_for_sum_list[of S _ us]
            reduced_word_for_imp_reduced_word[of S _ us] el_reduced[of S]
      by    auto
    with us ts show "tT"
      using reduced_word_for_sum_list[of S _ us] by (cases us) auto
  from assms show "T  T  S" using genby_genset_subset by fast

lemma special_subgroup_inj: "inj_on genby (Pow S)"
  using special_subgroup_int_S inj_on_inverseI[of _ "λW. WS"] by fastforce

lemma special_subgroup_genby_subset_ordering_iso:
  "subset_ordering_iso (Pow S) genby"
proof (unfold_locales, rule genby_mono, simp, rule special_subgroup_inj)
  fix X Y assume XY: "X  genby ` Pow S" "Y  genby ` Pow S" "XY"
  from XY(1,2) obtain TX TY
    where "TXPow S" "X = TX" "TYPow S" "Y = TY"
    by    auto
  hence "the_inv_into (Pow S) genby X = XS"
        "the_inv_into (Pow S) genby Y = YS"
    using the_inv_into_f_f[OF special_subgroup_inj] special_subgroup_int_S
    by    auto
  with XY(3)
    show  "the_inv_into (Pow S) genby X  the_inv_into (Pow S) genby Y"
    by    auto

lemmas special_subgroup_genby_rev_mono
  = OrderingSetIso.rev_ordsetmap[OF special_subgroup_genby_subset_ordering_iso]

lemma special_subgroup_word_length:
  assumes "TPow S" "wT"
  shows   "word_length T w = S_length w"
  from assms obtain ts where ts: "ts  lists T" "w = sum_list ts"
    using special_subgroup_eq_sum_list by auto
  with assms(1) obtain us where "us  ssubseqs ts" "S_reduced_for w us"
    using deletion_reduce[of ts] by fast
  with assms(1) ts(1) show ?thesis
    using     ssubseqs_lists[of ts] reduced_word_for_sum_list
              is_arg_min_size_subprop[of length "word_for S w" us "word_for T w"]
    unfolding reduced_word_for_def word_length_def
    by        fast

lemma S_subset_reduced_imp_S_reduced:
  "TPow S  reduced_word T ts  S_reduced ts"
  using reduced_word_for_lists reduced_word_for_lists[of T _ ts]
        reduced_word_for_length[of T "sum_list ts" ts] special_subgroup_eq_sum_list[of T]
        special_subgroup_word_length[of T "sum_list ts"]
  by    (fastforce intro: reduced_word_forI_length)

lemma smallest_genby: "TPow S  wT  reduced_letter_set S w  T"
  using genby_S_reduced_word_for_arg_min[of T]
        reduced_word_for_imp_reduced_word[of T w]
        S_subset_reduced_imp_S_reduced[of T "arg_min length (word_for T w)"]
        reduced_word_for_sum_list[of T] reduced_word_for_lists reduced_word_letter_set
  by    fastforce

lemma special_cosets_below_in:
  assumes "wW" "T  Pow S"
  shows   "𝒫.⊇(w +o T) = (R(Pow S).⊇T. {w +o R})"
proof (rule seteqI)
  fix A assume "A  𝒫.⊇(w +o T)"
  hence A: "A𝒫" "A  (w +o T)" by auto
  from A(1) obtain R w' where "RPow S" "A = w' +o R"
    using special_cosets_def by auto
  with A(2) assms(2) show "A  (R(Pow S).⊇T. {w +o R})"
    using genby_lcoset_subgroup_imp_eq_reps[of w T w' R]
          lcoset_eq_reps_subset[of w "T"]
          special_subgroup_genby_rev_mono[of T R]
    by    auto
  fix B assume "B  (R(Pow S).⊇T. {w +o R})"
  from this obtain R where R: " R  (Pow S).⊇T" "B = w +o R" by auto
  moreover hence "B  w +o T"
    using genby_mono elt_set_plus_def[of w] by auto
  ultimately show "B  special_cosets .⊇ (w +o T)"
    using assms(1) special_cosetsI by auto

lemmas special_coset_inj
  = comp_inj_on[OF special_subgroup_inj, OF inj_inj_on, OF lcoset_inj_on]

lemma special_coset_eq_imp_eq_gensets:
  " T1Pow S; T2Pow S; w1 +o T1 = w2 +o T2   T1=T2"
  using set_plus_rearrange2[of "-w1" w1 "T1"]
        set_plus_rearrange2[of "-w1" w2 "T2"]
        genby_lcoset_subgroup_imp_eq_reps[of 0 T1 "-w1+w2" T2]
        inj_onD[OF special_subgroup_inj]
  by    force

lemma special_subgroup_special_coset_subset_ordering_iso:
  "subset_ordering_iso (genby ` Pow S) ((+o) w)"
  show "a b. a  b  w +o a  w +o b" using elt_set_plus_def by auto
  show 2: "inj_on ((+o) w) (genby ` Pow S)"
    using lcoset_inj_on inj_inj_on by fast
  show  "a b. a  (+o) w ` genby ` Pow S 
          b  (+o) w ` genby ` Pow S 
          a  b 
          the_inv_into (genby ` Pow S) ((+o) w) a 
            the_inv_into (genby ` Pow S) ((+o) w) b"
    fix a b
    assume ab : "a  (+o) w ` genby ` Pow S" "b  (+o) w ` genby ` Pow S"
      and  a_b: "ab"
    from ab obtain Ta Tb
      where "TaPow S" "a = w +o Ta" "TbPow S" "b = w +o Tb"
      by    auto
    with a_b
      show  "the_inv_into (genby ` Pow S) ((+o) w) a 
              the_inv_into (genby ` Pow S) ((+o) w) b"
      using the_inv_into_f_eq[OF 2] lcoset_eq_reps_subset[of w "Ta" "Tb"]
      by    simp

lemma special_coset_subset_ordering_iso:
  "subset_ordering_iso (Pow S) ((+o) w  genby)"
  using special_subgroup_genby_subset_ordering_iso
  by    (fast intro: OrderingSetIso.iso_comp)

lemmas special_coset_subset_rev_mono =
  OrderingSetIso.rev_ordsetmap[OF special_coset_subset_ordering_iso]

lemma special_coset_below_in_subset_ordering_iso:
  "subset_ordering_iso ((Pow S).⊇T) ((+o) w  genby)"
  using special_coset_subset_ordering_iso by (auto intro: OrderingSetIso.iso_subset)

lemma special_coset_below_in_supset_ordering_iso:
  "OrderingSetIso (⊇) (⊃) (⊇) (⊃) ((Pow S).⊇T) ((+o) w  genby)"
  using special_coset_below_in_subset_ordering_iso OrderingSetIso.iso_dual by fast

lemma special_coset_pseudominimals:
  assumes "supset_pseudominimal_in 𝒫 X"
  shows   "w s. wW  sS  X = w +o S-{s}"
  from assms have "X𝒫" using supset_pseudominimal_inD1 by fast
  from this obtain w T where wT: "wW" "TPow S" "X = w +o T"
    using special_cosets_def by auto
  show ?thesis
  proof (cases "T=S")
    case True with wT(1,3) assms show ?thesis
      using genby_lcoset_el_reduce supset_pseudominimal_ne_bottom
      by    fast
    case False
    with wT(2) obtain s where s: "sS" "T  S-{s}" by fast
    from s(2) wT(1,3) assms have "X  w +o S-{s}"
      using genby_mono by auto
    moreover from assms wT(1) s(1) have "¬ X  w +o S-{s}"
      using special_cosetsI[of _ w]
            supset_pseudominimal_inD2[of 𝒫 X "w +o S-{s}"]
            lcoset_eq_reps[of w _ "S"]
            inj_onD[OF special_subgroup_inj, of "S-{s}" S]
      by    (auto simp add: special_cosets_bottom genby_lcoset_el_reduce)
    ultimately show ?thesis using wT(1) s(1) by fast

lemma special_coset_pseudominimal_in_below_in:
  assumes "wW" "TPow S" "supset_pseudominimal_in (𝒫.⊇(w +o T)) X"
  shows   "sS-T. X = w +o S-{s}"
  from assms obtain v s where vs: "vW" "sS" "X = v +o S-{s}"
    using special_cosets_has_bottom special_cosetsI[of T w]
    by    force
  from assms(3) have X: "X  w +o T"
    using supset_pseudominimal_inD1 by fast
  with vs(3) have 1: "X = w +o S-{s}"
    using genby_lcoset_subgroup_imp_eq_reps[of w T v "S-{s}"] by fast
  with X assms have "T  S-{s}"
    using special_cosetsI special_coset_subset_rev_mono[of T "S-{s}"]
    by    fastforce
  with vs(2) show ?thesis using 1 by fast

lemma exclude_one_is_pseudominimal:
  assumes "wW" "tS"
  shows   "supset_pseudominimal_in 𝒫 (w +o S-{t})"
proof (rule supset_pseudominimal_inI, rule special_cosetsI)
  show "w  W" by fact
  from assms have "w +o S - {t}  W"
    using genby_lcoset_el_reduce[of w] lcoset_eq_reps[of w _ W] 
          inj_onD[OF special_subgroup_inj, of "S-{t}" S]
    by    auto
  thus "w +o S - {t}  supset_bottom 𝒫"
    using special_cosets_bottom by fast
  fix X assume X: "X𝒫" "w +o S - {t}  X"
  with assms(1) have "X  (R(Pow S).⊇(S-{t}). {w +o R})"
    using subst[OF special_cosets_below_in, of w "S-{t}" "λA. XA"] by fast
  from this obtain R where R: "R  (Pow S).⊇(S-{t})" "X = w +o R" by auto
  from R(2) X(2) have "R  S-{t}" by fast
  with R(1) have "R=S" by auto
  with assms(1) R(2) show "X = supset_bottom 𝒫"
    using genby_lcoset_el_reduce special_cosets_bottom by fast
qed fast

lemma exclude_one_is_pseudominimal_in_below_in:
  " wW; TPow S; sS-T  
    supset_pseudominimal_in (𝒫.⊇(w +o T)) (w +o S-{s})"
  using special_cosets_has_bottom special_cosetsI
        exclude_one_is_pseudominimal[of w s]
        genby_mono[of T "S-{s}"]
          of 𝒫 "w +o T" "w +o S-{s}"
  by    auto

lemma glb_special_subset_coset:
  assumes   wTT': "w W" "T  Pow S" "T'  Pow S"
  defines   U: "U  T  T'  reduced_letter_set S w"
  shows     "supset_glbound_in_of 𝒫 T (w +o T') U"
proof (rule supset_glbound_in_ofI)

  from wTT'(2,3) U show "U  𝒫"
    using reduced_letter_set_subset[of S] special_subgroup_special_coset by simp

  show "supset_lbound_of T (w +o T') U"
  proof (rule supset_lbound_ofI)
    from U show "T  U" using genby_mono[of T U] by fast
    show "w +o T'  U"
      fix x assume "x  w +o T'"
      with wTT'(3) obtain y where y: "y  T'" "x = w + y"
        using elt_set_plus_def[of w] by auto
      with wTT'(1) U show "x  U"
        using in_genby_S_reduced_letter_set genby_mono[of _ U]
              genby_mono[of T' U] genby_add_closed[of w U y]
        by    auto


  fix X assume X: "X𝒫" "supset_lbound_of T (w +o T') X"
  from X(1) obtain v R where vR: "RPow S" "X = v +o R"
    using special_cosets_def by auto
  from X(2) have X': "X  T" "X  w +o T'"
    using supset_lbound_of_def[of _ _ X] by auto
  from X'(1) vR(2) have R: "X = R"
    using genby_0_closed genby_lcoset_el_reduce0 by fast
  with X'(2) have w: "wR" using genby_0_closed lcoset_refl by fast
  have "T'  R"
  proof (
    rule special_subgroup_genby_rev_mono, rule wTT'(3), rule vR(1), rule subsetI
    fix x assume "x  T'"
    with X'(2) R show "x  R"
      using elt_set_plus_def[of w "T'"] w genby_uminus_add_closed[of "w" R "w+x"]
      by    auto
  with X'(1) wTT'(2) vR(1) show "UX"
    using special_subgroup_genby_rev_mono[of T R] w smallest_genby U R
          genby_mono[of _ R]
    by    simp


lemma glb_special_subset_coset_ex:
  assumes   "w W" "T  Pow S" "T'  Pow S"
  shows     "B. supset_glbound_in_of 𝒫 T (w +o T') B"
  using     glb_special_subset_coset[OF assms]
  by        fast

lemma special_cosets_have_glbs:
  assumes "X𝒫" "Y𝒫"
  shows   "B. supset_glbound_in_of 𝒫 X Y B"
  from assms obtain wx Tx wy Ty
    where X: "wx  W" "Tx  Pow S" "X = wx +o Tx"
    and   Y: "wy  W" "Ty  Pow S" "Y = wy +o Ty"
    using special_cosets_def
    by    auto
  from X(1,2) Y(1,2) obtain A
    where A: "supset_glbound_in_of 𝒫 Tx ((-wx+wy) +o Ty) A"
    using genby_uminus_add_closed[of wx] glb_special_subset_coset_ex by fastforce
  from X(1,3) Y(3) have "supset_glbound_in_of 𝒫 X Y (wx +o A)"
    using supset_glbound_in_of_lcoset_shift[OF A, of wx]
    by    (auto simp add: set_plus_rearrange2 special_cosets_lcoset_shift)
  thus ?thesis by fast

end (* context PreCoxeterSystemWithDeletion *)

subsection ‹Coxeter systems›

subsubsection ‹Locale definition and transfer from the associated free group›

text ‹
  Now we consider groups generated by elements of order two with an additional assumption to ensure
  that the natural correspondence between the group @{term W} and the group presentation on the
  generating set @{term S} and its relations is bijective. Below, such groups will be shown to
  satisfy the deletion condition.

locale CoxeterSystem = PreCoxeterSystem S
  for S      :: "'w::group_add set"
+ assumes induced_id_inj: "inj_on induced_id G"

lemma (in PreCoxeterSystem) CoxeterSystemI:
  assumes "g. gG  induced_id g = 0  g=0"
  shows   "CoxeterSystem S"
  from assms have "GroupIso G induced_id"
    using GroupWithGeneratorsRelators_S_R
    by    (fast intro: GroupHom.isoI)
  thus "inj_on induced_id G" using GroupIso.inj_on by fast

context CoxeterSystem

abbreviation "inv_induced_id  GroupPresentation.inv_induced_id S R"

lemma GroupPresentation_S_R: "GroupPresentation S R"
  by  (
        intro_locales, rule GroupWithGeneratorsRelators_S_R,
        unfold_locales, rule induced_id_inj

lemmas inv_induced_id_sum_list =
  GroupPresentation.inv_induced_id_sum_list_S[OF GroupPresentation_S_R]

end (* context CoxeterSystem *)

subsubsection ‹The deletion condition is necessary›

text ‹
  Call an element of @{term W} a reflection if it is a conjugate of a generating element (and so
  is also of order two). Here we use the action of words over @{term S} on such reflections to show
  that Coxeter systems satisfy the deletion condition.

context CoxeterSystem

abbreviation "induced_signed_lconjperm 
  GroupByPresentationInducedFun.induced_hom S P signed_lconjpermutation"

definition flipped_reflections :: "'w  'w set"
  where "flipped_reflections w 
          {t. induced_signed_lconjperm (inv_induced_id (-w)) 
            (t,True) = (rconjby w t, False)}"

lemma induced_signed_lconjperm_inv_induced_id_sum_list:
  "ss  lists S  induced_signed_lconjperm (inv_induced_id (sum_list ss)) =
          sum_list (map signed_lconjpermutation ss)"
  by  (simp add:
        inv_induced_id_sum_list Abs_freelist_in_FreeGroup
          OF GroupByPresentationInducedFun_S_R_signed_lconjaction

lemma induced_signed_eq_lconjpermutation:
  "ss  lists S 
    permutation (induced_signed_lconjperm (inv_induced_id (sum_list ss))) =
      signed_list_lconjaction ss"
proof (induct ss)
  case Nil
  have "permutation (induced_signed_lconjperm (inv_induced_id (sum_list []))) = id"
    using induced_signed_lconjperm_inv_induced_id_sum_list[of "[]"]
    by    simp
  thus ?case by fastforce
  case (Cons s ss)
  from Cons(2)
    have "induced_signed_lconjperm (inv_induced_id (sum_list (s#ss))) =
            signed_lconjpermutation s + sum_list (map signed_lconjpermutation ss)"
    using induced_signed_lconjperm_inv_induced_id_sum_list[of "s#ss"]
    by    simp
  with Cons(2) have
    "permutation (induced_signed_lconjperm (inv_induced_id (sum_list (s#ss)))) =
      permutation (signed_lconjpermutation s) 
        permutation (induced_signed_lconjperm (inv_induced_id (sum_list ss)))"
    using plus_permutation.rep_eq induced_signed_lconjperm_inv_induced_id_sum_list
    by    simp
  with Cons show ?case
    using bij_signed_lconjaction[of s] Abs_permutation_inverse by fastforce

lemma flipped_reflections_odd_lconjseq:
  assumes "sslists S"
  shows   "flipped_reflections (sum_list ss) = {t. odd (count_list (lconjseq ss) t)}"
proof (rule seteqI)
  fix t assume "t  flipped_reflections (sum_list ss)"
  moreover with assms
    have  "snd (signed_list_lconjaction (rev ss) (t,True)) = False"
    using flipped_reflections_def genset_order2_add uminus_sum_list_order2
          induced_signed_eq_lconjpermutation[of "rev ss"]
    by    force
  ultimately show "t  {t. odd (count_list (lconjseq ss) t)}"
    using assms flipped_reflections_def genset_order2_add
          signed_list_lconjaction_snd[of "rev ss"]
    by    auto
  fix t assume t: "t  {t. odd (count_list (lconjseq ss) t)}"
  with assms
    have  "signed_list_lconjaction (rev ss) (t,True) =
            (rconjby (sum_list ss) t, False)"
    using genset_order2_add signed_list_lconjaction_snd[of "rev ss"]
          signed_list_lconjaction_fst[of "rev ss"]
          uminus_sum_list_order2[of ss, THEN sym]
    by    (auto intro: prod_eqI)
  with t assms show "t  flipped_reflections (sum_list ss)"
    using induced_signed_eq_lconjpermutation[of "rev ss"] genset_order2_add
          uminus_sum_list_order2 flipped_reflections_def
    by    fastforce

lemma flipped_reflections_in_lconjseq:
  "sslists S  flipped_reflections (sum_list ss)  set (lconjseq ss)"
  using flipped_reflections_odd_lconjseq odd_n0 count_notin[of _ "lconjseq ss"]
  by    fastforce

lemma flipped_reflections_distinct_lconjseq_eq_lconjseq:
  assumes "sslists S" "distinct (lconjseq ss)"
  shows   "flipped_reflections (sum_list ss) = set (lconjseq ss)"
  from assms(1) show "flipped_reflections (sum_list ss)  set (lconjseq ss)"
    using flipped_reflections_in_lconjseq by fast
  show "flipped_reflections (sum_list ss)  set (lconjseq ss)"
    fix t assume "t  set (lconjseq ss)"
    moreover with assms(2) have "count_list (lconjseq ss) t = 1"
       by (simp add: distinct_count_list)
    ultimately show "t  flipped_reflections (sum_list ss)"
      using assms(1) flipped_reflections_odd_lconjseq lconjseq_reflections
      by    fastforce

lemma flipped_reflections_reduced_eq_lconjseq:
  "S_reduced ss  flipped_reflections (sum_list ss) = set (lconjseq ss)"
  using reduced_word_for_lists[of S] S_reduced_imp_distinct_lconjseq
  by    fast

lemma card_flipped_reflections:
  assumes "wW"
  shows "card (flipped_reflections w) = S_length w"
  define ss where "ss = arg_min length (word_for S w)"
  with assms have "S_reduced_for w ss"
    using genby_S_reduced_word_for_arg_min by simp
  thus ?thesis
    using reduced_word_for_sum_list flipped_reflections_reduced_eq_lconjseq
          S_reduced_imp_distinct_lconjseq distinct_card length_lconjseq[of ss]
    by    fastforce

end (* context CoxeterSystem *)

sublocale CoxeterSystem < PreCoxeterSystemWithDeletion
  fix ss assume ss: "ss  lists S" "¬ S_reduced ss"
  define w where "w = sum_list ss"
  with ss(1)
    have  "distinct (lconjseq ss)  card (flipped_reflections w) = length ss"
    by    (simp add:
            flipped_reflections_distinct_lconjseq_eq_lconjseq distinct_card
  moreover from w_def ss have "length ss > S_length w" using word_length_lt by fast
  moreover from w_def ss(1) have "card (flipped_reflections w) = S_length w"
    using special_subgroup_eq_sum_list card_flipped_reflections by fast
  ultimately have "¬ distinct (lconjseq ss)" by auto
  with w_def ss
    show  "a b as bs cs. ss = as @ [a] @ bs @ [b] @ cs 
            sum_list ss = sum_list (as @ bs @ cs)"
    using deletion'
    by    fast

subsubsection ‹The deletion condition is sufficient›

text ‹
  Now we come full circle and show that a pair consisting of a group and a generating set of
  order-two elements that satisfies the deletion condition affords a presentation that makes it a
  Coxeter system.

context PreCoxeterSystemWithDeletion

lemma reducible_by_flipping:
  "ss  lists S  ¬ S_reduced ss 
    xss as t bs. flip_altsublist_chain (ss # xss @ [as@[t,t]@bs])"
proof (induct ss)
  case (Cons s ss)
  show ?case
  proof (cases "S_reduced ss")
    case True
    define w where "w = sum_list ss"
    with True have ss_red_w: "reduced_word_for S w ss" by fast
    moreover from Cons(2) have "sS" by simp
    ultimately obtain as bs where asbs: "reduced_word_for S w (s#as@bs)"
      using Cons(3) exchange by fast
    show ?thesis
    proof (cases "w=0")
      case True with asbs show ?thesis
        using reduced_word_for_0_imp_nil by fast
      case False
      from this obtain xss where "flip_altsublist_chain (ss # xss @ [s#as@bs])"
        using ss_red_w asbs reduced_word_problem by fast
      hence "flip_altsublist_chain (
              (s#ss) # map ((#) s) xss @ [[]@[s,s]@(as@bs)]
        using flip_altsublist_chain_map_Cons_grow by fastforce
      thus ?thesis by fast
    case False
    with Cons(1,2) obtain xss as t bs
      where "flip_altsublist_chain (
              (s#ss) # map ((#) s) xss @ [(s#as)@[t,t]@bs]
      using flip_altsublist_chain_map_Cons_grow
      by    fastforce
    thus ?thesis by fast
qed (simp add: nil_reduced_word_for_0)

lemma freeliftid_kernel':
  "ss  lists S  sum_list ss = 0  Abs_freelist ss  Q"
proof (induct ss rule: length_induct)
  fix ss
  assume step: "ts. length ts < length ss  ts  lists S 
                sum_list ts = 0  Abs_freelist ts  Q"
  and set_up: "ss  lists S" "sum_list ss = 0"
  show "Abs_freelist ss  Q"
  proof (cases "ss=[]")
    case True thus ?thesis
      using genby_0_closed[of "wFreeGroup S. lconjby w ` P'"]
      by    (auto simp add: zero_freeword.abs_eq)
    case False
    with set_up obtain xss as t bs
      where xss: "flip_altsublist_chain (ss # xss @ [as@[t,t]@bs])"
      using sum_list_zero_nreduced reducible_by_flipping[of ss]
      by    fast
    with set_up
      have  astbs:  "length (as@[t,t]@bs) = length ss"
                    "as@[t,t]@bs  lists S"
                    "sum_list (as@[t,t]@bs) = 0"
      using flip_altsublist_chain_length[of ss xss "as@[t,t]@bs"]
            flip_altsublist_chain_sum_list[of ss xss "as@[t,t]@bs"]
            flip_altsublist_chain_lists[of ss xss "as@[t,t]@bs"]
      by    auto
    have listsS: "as  lists S" "tS" "bslists S" using astbs(2) by auto
    have "sum_list as + (t + t + sum_list bs) = 0"
      using astbs(3) by (simp add: add.assoc)
    hence "sum_list (as@bs) = 0"
      using listsS(2) by (simp add: genset_order2_add)
    moreover have "length (as@bs) < length ss" using astbs(1) by simp
    moreover have "as@bs  lists S" using listsS(1,3) by simp
    ultimately have "Abs_freelist (as@bs)  Q" using step by fast
    hence "Abs_freelist as + pair_relator_freeword t t +
            (- Abs_freelist as + (Abs_freelist as + Abs_freelist bs))  Q"
      using listsS(1,2) lconjby_Abs_freelist_relator_freeword[of t t as]
      by    (simp add: Abs_freelist_append[THEN sym] add.assoc[THEN sym])
    hence "Abs_freelist as + Abs_freelist [t,t] + Abs_freelist bs  Q"
      using listsS(2) by (simp add: S_relator_freeword Abs_freeletter_add)
    thus ?thesis
      using Abs_freelist_append_append[of as "[t,t]" bs]
            rev_flip_altsublist_chain[OF xss]
            flip_altsublist_chain_G_in_Q[of "as@[t,t]@bs" "rev xss" ss]
      by    simp

lemma freeliftid_kernel:
  assumes "c  FreeGroup S" "freeliftid c = 0"
  shows   "cQ"
  from assms(2) have "freeliftid (Abs_freeword (freeword c)) = 0"
    by (simp add: freeword_inverse)
  with assms(1) have "sum_list (map fst (freeword c)) = 0"
    using FreeGroup_def freeword freeliftid_Abs_freeword_conv_sum_list by fastforce
  with assms(1) show ?thesis
    using FreeGroup_def freeliftid_kernel'[of "map fst (freeword c)"]
    by    fastforce

lemma induced_id_kernel:
  "c  FreeGroup S  induced_id (FreeGroup S|c|Q) = 0  cQ"
  by  (simp add:
          OF GroupByPresentationInducedFun_S_P_id

theorem CoxeterSystem: "CoxeterSystem S"
proof (