removed an unnecessary restriction in exercise 7.5
forgot part of the change
linebreak added
made the base case clear
eliminate premature "Theorem", see #519
add exercise from Marc, close #498
unmarked errata for 810aa94
Merge pull request #518 from sattlerc/master

removed an unnecessary restriction in exercise 7.5
Merge pull request #526 from mikeshulman/lem2.10.5ex

add exercise from Marc, close #498
Merge pull request #523 from mikeshulman/firsttime

eliminate premature "Theorem", see #519
marked errata for 810aa94