X_SKOLEM_CONV : (term -> conv)
Introduces a user-supplied Skolem function.
X_SKOLEM_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 !x1...xn. ?y. P. Given these arguments, X_SKOLEM_CONV returns the theorem:
   |- (!x1...xn. ?y. P) = (?f. !x1...xn. tm[f x1 ... xn/y])
which expresses the fact that a skolem function f of the universally quantified variables x1...xn may be introduced in place of the the existentially quantified value y.
X_SKOLEM_CONV f tm fails if f is not a variable, or if the input term tm is not a term of the form !x1...xn. ?y. P, or if the variable f is free in tm, or if the type of f does not match its intended use as an n-place curried function from the variables x1...xn to a value having the same type as y.
HOL  Kananaskis-14