BibTeX: 
@article{Delta_System_LemmaAFP,
author = {Pedro Sánchez Terraf},
title = {Cofinality and the Delta System Lemma},
journal = {Archive of Formal Proofs},
month = dec,
year = 2020,
note = {\url{https://isaafp.org/entries/Delta_System_Lemma.html},
Formal proof development},
ISSN = {2150914x},
}
