According to help(=) and help(==), it follows that X = Y is true if and only if X== Y is true provided that X and Y are variable free (ground). Now I confirm that =/2 on ground terms exactly corresponds to the notion of bi-similar. I am glad to realize this relation clearly. I noticed for a long time that they are similar, but not upto “exactly.” Indeed =/2 means unifiabe , that is, identifiable. Just a monologue, but hope there are those who shares this as fresh rediscovery.