add simple gensym library

src/prolog/lib/gensym.pl

@@ -0,0 +1,29 @@
+
+:- module(gensym, [gensym/2,
+		   reset_gensym/1]).
+
+:- use_module(library(error)).
+:- use_module(library(lists)).
+:- use_module(library(non_iso)).
+:- use_module(library(si)).
+
+append_id(Base, UniqueID, Unique) :-
+    atom_chars(Base, BaseChars),
+    number_chars(UniqueID, IDChars),
+    append(BaseChars, IDChars, AtomChars),
+    atom_chars(Unique, AtomChars).
+
+gensym(Base, Unique) :-
+    must_be(var, Unique),
+    atom_si(Base),    
+    (  bb_get(Base, UniqueID0) ->
+       UniqueID is UniqueID0 + 1,
+       bb_put(Base, UniqueID),
+       append_id(Base, UniqueID, Unique)
+    ;  bb_put(Base, 1),
+       append_id(Base, 1, Unique)
+    ).
+
+reset_gensym(Base) :-
+    atom_si(Base),
+    bb_put(Base, 0).