| ⊦ n < # y → (perm(x, y[n, a]) ↔ a ∈ x ∧ perm(x -1l a, y -1l y[n])) |
n < # y ⊦ perm(x, y[n, a]) ↔ a ∈ x ∧ perm(x -1l a, y -1l y[n]), y = []
Used simplifier rules: l: ⊦ # x = 0 ↔ x = []; from specification list-data%f: ⊦ m < n → n ≠ 0; (forward) from specification nat-basic1