Prolog - Generate Unique Symbol -

in prolog, aware of gensym predicate:

gensym(g, x). x = g1. gensym(g, x). x = g2. gensym(g, x). x = g3. 

but docs say: "note no guarantee atom unique in system."

i looking predicate exactly, if gensymming new symbol, not match 1 used in same program.