typos in Lemma 6.12.7, close #573
unmarked errata for 6dce346
explain more about justifications for equalities, close #578
typo in proof of lemma 2.7.2
unmarked errata for ba035f6
oops
Merge pull request #575 from mikeshulman/lemma6.12.7

typos in Lemma 6.12.7, close #573
Merge pull request #579 from mikeshulman/justifications

explain more about justifications for equalities, close #578
Merge pull request #580 from mikeshulman/refl

typo in proof of lemma 2.7.2
Mark Errata (auto)