* The fresh(S:Sort) predicate needs to be explained better.

Mea culpa: It is explained better in Tutorial 4 (on IMP++), Lesson 2.  I did
the README files for Tutorial 4 before those for Tutorial 3, and I mistakenly
thought that fresh has already been explained.

* Once the new symbolic rewrite engine is available, I should replace

  syntax K ::= env(...)

with

  syntax KItem ::= env(...)

They are equivalent, but the latter is cleaner, more efficient, and more
ameanable for symbolic reasoning.
