The standard ordering of prolog terms is shown for compare/3.
Success: X @< 1.0. (gives X = _g68) 0.0 @< 1. 2.0 @< 1. "a" @< a. atoma @< atomb. [a|b] @< [a,b]. b(1) @< a(1,1). a(1,2,3,4.0) @< a(1,2,3,0). Fail: 1.0 @< X. atomb @< atoma. f(1,1) @< f(1).