P_PSKOLEM_CONV takes two arguments. The first is a variable f, which
must range over functions of the appropriate type, and the second is a term of
the form !p1...pn. ?q. t (where pi and q may be pairs).
Given these arguments, P_PSKOLEM_CONV returns the theorem:
|- (!p1...pn. ?q. t) = (?f. !p1...pn. tm[f p1 ... pn/q])
which expresses the fact that a skolem function f of the
universally quantified variables p1...pn may be introduced in place of the
the existentially quantified pair p.