add crossref, close #351
Merge pull request #353 from mikeshulman/crossref

add crossref, close #351
Correct a whitespace error
Correct "p" to "r" in the proof of Lemma 3.11.7
Correct a dislocated comma
Merge pull request #354 from rwbarton/master

Correct a whitespace error
Merge pull request #355 from rwbarton/lemma-3-11-7

Correct "p" to "r" in the proof of Lemma 3.11.7
Merge pull request #358 from rwbarton/chapter-5-8

Correct a dislocated comma
fix typo around Theorem 2.12.5, close #350
Remove some stray commas
Reunite a long sentence
Fix a typo: "entires"
Merge pull request #364 from rwbarton/exercise-7.2

Fix a typo: "entires"
Merge pull request #362 from rwbarton/section-7.3

Section 7.3 punctuation fixes