improved \prd and \sm magic
manual reflowing of W_s
Merge pull request #490 from mikeshulman/prdmagic

improved \prd and \sm magic
fix statement of ex 4.2
unmarked errata for a9c974c
solution to ex 4.2
correct references in ex 4.2 solution
Merge pull request #496 from mikeshulman/ex4.2

Fix exercise 4.2, close #495
fix a misuse of \refl