⊦ ¬ a ∈ x → x ++ a = a + x
¬ ¬ a ∈ x ⊦ perm(x, y) → perm(x ++ a, y ++ a)
¬ a ∈ x ⊦ perm(x, y) → perm(a + x, y ++ a)
Used terms: a, x