predicate fzn_inverse(array[int] of var int: f,
                  array[int] of var int: invf) =
    forall(i in index_set(f)) (
        f[i] in index_set(invf) /\
        (invf[f[i]] == i)
    ) /\
    forall(j in index_set(invf)) (
        invf[j] in index_set(f) /\
        (f[invf[j]] == j)
    );