Specializes a theorem, with type instantiation if necessary.
DESCRIPTION
This rule specializes a quantified variable as does SPEC; it differs
from it in also instantiating the type if needed:
A |- !x:ty.tm
----------------------- ISPEC "t:ty'"
A |- tm[t/x]
(where t is free for x in tm, and ty' is an instance
of ty).
FAILURE
ISPEC fails if the input theorem is not universally quantified, if
the type of the given term is not an instance of the type of the
quantified variable, or if the type variable is free in the
assumptions.