Solution to Exercise 3.1 and two preparing lemmas added.
Merge pull request #445 from marcbezem/master

Solution to Exercise 3.1 and two preparing lemmas added.
Move some [Require]s to the top of a file

It's bad practice to have them in the middle of the file.  Also, it
makes my coq-bug-finder run afoul of
https://coq.inria.fr/bugs/show_bug.cgi?id=3556 when tracking down a bug
in [apply] in the new version of trunk with #440.