Undocumented predicates

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

 beta_subst(Arg1, Arg2, Arg3)

 normal_form(Arg1, Arg2)

 fresh_bind(Arg1, Arg2)

 fresh_goal(Arg1, Arg2, Arg3)

 fresh_call(Arg1, Arg2)