Loop detection: before expanding a haz formula, see whether it is a duplicate
and, if so, delete it.
Recursion detection: transitivity and similar rules, when applied, put the
new formulae at the end of a branch and not at the front (in effect).
How to build TFL and run the Unify example.
1. Invoke the current version of Isabelle-HOL.
2. use "sys.sml";
3. cd examples/Subst
4. use "ROOT.ML";