Success: atom == atom. 1 == 1. X == X. (gives X = _g70) X = 1, Y = 1, Y == X. (gives X = 1, Y = 1) X = Y, X == Y, Y == X. (gives Y = _g80, X = _g80) [ a,b| [ ] ] == [ a,b ]. f(1,2) == f(1,2). Fail: atom == neutron. atom == X. 1 == 1.0. X == Y. [a|b] == [a,b]. [a|X] == [a,X].