BibTeX: 
@article{Kuratowski_Closure_ComplementAFP,
author = {Peter Gammie and Gianpaolo Gioiosa},
title = {The Kuratowski ClosureComplement Theorem},
journal = {Archive of Formal Proofs},
month = oct,
year = 2017,
note = {\url{https://isaafp.org/entries/Kuratowski_Closure_Complement.html},
Formal proof development},
ISSN = {2150914x},
}
