Specializes a theorem zero or more times, with type instantiation if necessary.
DESCRIPTION
ISPECL is an iterative version of ISPEC
A |- !x1...xn.t
---------------------------- ISPECL [t1,...,tn]
A |- t[t1,...tn/x1,...,xn]
(where ti is free for xi in tm).
FAILURE
ISPECL fails if the list of terms is longer than the number of
quantified variables in the term, if the type instantiation fails, or
if the type variable being instantiated is free in the assumptions.