-- added combinator reduction axioms (typed and untyped) for HOL goals.
-- combined make_nnf functions for HOL and FOL goals.
-- hypothesis of goals are now also skolemized by inference.
-- removed "check_is_fol" from "make_nnf" so that the NNF procedure doesn't check whether a thm is FOL.
-- added "check_is_fol_term", which is the same as "check_is_fol", but takes a "term" as input.
-- added "check_is_fol" and "check_is_fol_term" into the signature.