Reverse.Ordcompare a b compares a and b for ordering.
compare a b < 0 if a precedes b (denoted a < b),compare a b = 0 if a is equal to b (denoted a = b),compare a b > 0 if a succeeds b (denoted a > b).This should satisfy the following properties:
(compare a b <= 0 || compare b a >= 0) = true,(compare a b <= 0) = true and (compare b c <= 0) = true, then (compare a c <= 0) = true,(compare a a = 0) = true,(compare a b <= 0) = true and (compare a b >= 0) = true then (compare a b = 0) = true.