The Stacks project

Add mono+epi = iso for abelian cats

Thanks to Xueping Huang
https://stacks.math.columbia.edu/tag/00ZX#comment-8377

Add internal reference to definition kernel

Thanks to Hans Schoutens
https://stacks.math.columbia.edu/tag/00ZX#comment-6211

Fix a typo

Thanks to Darij Grinberg

Small changes to homology

homology.tex: fill in two more omitted proofs

Lemma \ref{lemma-abelian-opposite} (I hope I didn't miss any
subtleties here) and Lemma \ref{lemma-characterize-injective}.

Note that the latter still applies in any preadditive category
as long as we assume that Ker(f) resp Coker(f) exist.
Maybe it's worth generalizing it?

Started tags infrastructure

	new file:   scripts/add_tags.py
	modified:   scripts/functions.py
	new file:   tags/initial_tags
	new file:   tags/tags

Typos in homology.tex

Large set of changes involving:

	New file homology.tex for homological algebra
	Fixes in file injectives.tex
	Typos by Zach Maddock
	Example by Alexander Palen Ellis
	Proof of commutative square by Kristen Hendricks
	Proof of K_0 local ring by Ian Whitehead
	Proof of standard cover lemma by Andrew Fanoe