The standard ordering of prolog terms is given for compare/3.
Success: X @=< 1.0. (gives X = _g68) 1.0 @=< 0. 0 @=< "zero". same @=< same. diffa @=< diffb. [a|b] @=< [a,b]. [a,b|X] @=< [a,b,c]. (gives X = _g90) f(100) @=< f(0,0). a(100) @=< b(1). Fail: 1.0 @=< X. 0 @=< 1.0. atom @=< "atom". a(1,2,3) @=< a(1,2,X).