| n < # y, perm(x -1l a, y -1l y[n]), perm(y -1l y[n], x -1l a), a ∈ x ⊦ y = [], perm(x, y[n, a]) |
⊦ perm(x -1l a, y -1l a) ∧ a ∈ x ∧ a ∈ y → perm(x, y)
n < # y, perm(y -1l y[n], x -1l a), perm(x -1l a, y -1l y[n]), a ∈ x ⊦ y = [], perm(x, y[n, a]), perm(x -1l a, y[n, a] -1l a) ∧ a ∈ x ∧ a ∈ y[n, a]
Used terms: a, y[n, a], x