m < # y, n < # y ⊦ perm(x, y[m, y[n]][n, y[m]]) ↔ perm(x, y), y = []
Used simplifier rules: ⊦ # x = 0 ↔ x = []; (s)%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic