"backtrack unification", that is, unification
that allows more than one unifier for a pair of terms, and computes
the unifiers incrementally by backtracking.  As I write this,
we support associative commutative (AC) operations and 
commutative/symmetric (C) operations.  Symbols are declared to be
AC with set_assoc_comm() and C with set_commutative().
The use of Terms and Contexts is similar to ordinary unification,
except that the means for undoing substitutions is different.

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