fixed missing copyright info during rebasing
removed an opt. that doesn't apply to unification because variables of a rule can end up in the constraint rather than in substitution form; and they must be renamed
fixed the computation of unbound variables of a rule
minor refactorings; more comments
a few things: 1) renamed variables when neccessary; 2) applied owise'' rule conservatively; 3) revised assertion message
fixed a latent bug due to variable interference