close #448
close #450
unmarked errata for 88ab6d2
unmarked errata for 7d525ce
close #463
unmarked errata for e51d87f
close #445
unmarked errata for f8dab6a
add a bit of detail to the proof of 4.7.1
correct the proof of 2.11.1
refer back to 2.11.1 from 2-out-of-6 exercise
unmarked errata for 5702513
fix to 1.4
close #464
unmarked errata for 6531437
Merge pull request #467 from mikeshulman/lemma4.2.12

close #448
Merge pull request #468 from mikeshulman/theorem4.4.3

Theorem4.4.3
Merge pull request #469 from mikeshulman/theorem2.11.4

Theorem2.11.4
Merge pull request #470 from mikeshulman/lefttoright

Left-to-right
Merge pull request #471 from mikeshulman/theorem2.11.1

Fix proof of Theorem 2.11.1, close #461