n < # y, perm(x, y[n, a]) ⊦ y = [], a ∈ x ∧ perm(x -1l a, y -1l y[n])
n < # y, perm(x -1l a, y -1l y[n]), a ∈ x ⊦ y = [], perm(x, y[n, a])