a ∈ x, perm(x -1l a, x), a ∈ x ∨ a ∈ x -1l a → #oc(a, x) = #oc(a, x -1l a), ∀ a0. a0 ∈ x ∨ a0 ∈ x -1l a → #oc(a0, x) = #oc(a0, x -1l a) ⊦
Used terms: a