Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 fol_cnf(Arg1, Arg2)

 factoring(Arg1, Arg2)

 elim_variant(Arg1, Arg2)

 cls_union(Arg1, Arg2, Arg3)

 variant_memberchk(Arg1, Arg2)

 cnf_normal(Arg1, Arg2)

 cnf_variant_normal(Arg1, Arg2)

 ltr_check(Arg1, Arg2, Arg3, Arg4)

 cls_sort(Arg1, Arg2)

 tautology(Arg1)

 unord_tautology(Arg1)

 flip_polarity(Arg1, Arg2)