When applied to theorems A1 |- !x::P. Q[x] and A2 |- P x', the
derived inference rule RESQ_MATCH_MP matches x' to x by instantiating
free or universally quantified variables in the first theorem (only),
and returns a theorem A1 u A2 |- Q[x'/x]. Polymorphic types are also
instantiated if necessary.
A1 |- !x::P.Q[x] A2 |- P x'
-------------------------------------- RESQ_MATCH_MP
A1 u A2 |- Q[x'/x]