remove vestigial prolog/ directory (#444)

src/lib/diag.pl

@@ -0,0 +1,14 @@
+:- module(diag, [wam_instructions/2]).
+
+:- use_module(library(error)).
+
+wam_instructions(Clause, Listing) :-
+    (  nonvar(Clause) ->
+       Clause = Name / Arity,
+       must_be(atom, Name),
+       must_be(integer, Arity),
+       (  Arity >= 0 -> '$wam_instructions'(Name, Arity, Listing)
+       ;  throw(error(domain_error(not_less_than_zero, Arity), wam_instructions/2))
+       )
+    ;  throw(error(instantiation_error, wam_instructions/2))
+    ).