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