6.4.2 isn't using funext, close #640
errata for ec1373a
remark 1+n=n+1 is not judgmental
clarify exercises 5.2 and 5.3, close #639
errata for bd29d77
close #638
errata for 43ea0a8
Merge pull request #642 from mikeshulman/recurrence

close #639
Merge pull request #643 from mikeshulman/lemma6.4.2

6.4.2 isn't using funext, close #640
Merge pull request #644 from mikeshulman/thm2.11.3

close #638
Mark Errata (auto)