| ⊦ m < # y ∧ n < # y ∧ a = y[n] ∧ b = y[m] → (perm(x, y[m, a][n, b]) ↔ perm(x, y)) |
n < # y, m < # y ⊦ perm(x, y[m, y[n]][n, y[m]]) ↔ perm(x, y), y = []
Used simplifier rules: l: ⊦ # x = 0 ↔ x = []; from specification list-data%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1