S is S1 + S2 strings concatenation
R is str_length(S) finds the length of the given string
equal(S1, S2) compares strings with case (the same as =:= )
equal(S1, S2, no_case) compares strings (with no case)
less_than(S1, S2) compares strings with case (the same as < )
less_than(S1, S2, no_case) compares strings (with no case)

For comparing strings you can use also the arithmetic compare predicates. For them look at Comparison