predicate fzn_roots(array[int] of var int: x, var set of int: s,
                    var set of int: t) =
    % All values in 's' must map to a value in 't'.
    forall(i in ub(s)) (
        i in s -> x[i] in t
    ) /\
    forall(i in ub(t)) (
        i in t -> forall(j in index_set(x)) (x[j] = i -> j in s )
    );