fix 6.10.12, close #574 and #577
fix section 8.1 to match new 6.10.12
an exercise for the other definition of Z
unmarked errata for de14e6b
one more erratum
yet another erratum
improve notation
Merge branch 'master' into integers

Conflicts:
	errata.tex
Merge pull request #583 from mikeshulman/integers

fix 6.10.12, close #574 and #577