close #513
fix #514
parentheses in (k+1)-type
unmarked errata for a757864
unmarked errata for 4c4aee9
Merge pull request #525 from mikeshulman/lemma8.6.1

Lemma8.6.1
Merge pull request #524 from mikeshulman/lemma8.6.2

close #513
Typo in sec:equality-semigroups
Errata for 53a86bc, unmarked
Merge pull request #528 from TimRichter/typoeqsemigrp

Typo in sec_equality-semigroups
mark errata
strong from weak funext, close #507
unmarked errata for 8b15ad0
formatting
add errata.pdf to .gitignore
new exercise from Marc, close #521
swap encode and decode in 7.3.12, close #517
unmarked errata for aa9b06e
close #485
unmarked errata for b7c26e3