X = Y % X and Y are unifiable T1 == T2 % True if T1 and T2 are identical (names of variables are the same) T1 \== T2 % True if T1 and T2 are not identical E1 =:= E2 % True if values of expressions E1 and E2 are equal E1 =\= E2 % True if values of expressions E1 and E2 are not equal E1 < E2 % True if numeric value of E1 is < numeric value of E2 Similar for operators =<, >, and >= positive(N) :- N>0. non_zero(N) :- N<0 ; N>0.