predicate fzn_range_reif(array[int] of var int: x, var set of int: s,
                         var set of int: t,
                         var bool: b) =
        b <-> (
          % All values in 's' must map to a value in 't'.
          forall(i in ub(s)) (
              i in s -> x[i] in t
          ) /\
          % All values in 't' must be mapped from a value in 's'.
          forall(i in ub(t)) (
              i in t -> exists(j in ub(s)) ( j in s /\ x[j] == i )
          )
        );
