the condition on subsumption was too relaxed, in the case of: P(x) :- P(x) subsumes P(y) :- P(z), y and z are implicitly unified if the free variables in the range of the substitution are excluded from subsequent substitutions.

git-svn-id: http://gapt.googlecode.com/svn/trunk/source@1895 85d21ef8-add0-11de-ae77-5bfb9c5a3f34