mention Sigmas as subtypes in section 1.11
Explain equalities in Remark 1.12.1, close #516
unmarked errata for c4381c0
oops, use cleveref
double oops
Merge pull request #599 from mikeshulman/remark1.12.1

fix Remark 1.12.1, close #516
Mark Errata (auto)