align defn of ackermann function with wikipedia, close #360
Clarify in what sense variables in 1.11 are "unnamed", #347
add 1e868fa to errata
Add fe4641b to errata
Mention why 4.9.2 uses univalence, close #366
add aba76c8 to errata
Merge pull request #369 from mikeshulman/nontrivial4.9.2

Mention why 4.9.2 uses univalence, close #366
Merge pull request #367 from mikeshulman/ackermann

align defn of ackermann function with wikipedia, close #360
Merge pull request #368 from mikeshulman/unnamed

Clarify in what sense variables in 1.11 are "unnamed", #347