| ⊦ perm(x, y + a ') ↔ a ∈ x ∧ perm(x -1l a, y) |
⊦ perm(x, [] + a ') ↔ a ∈ x ∧ perm(x -1l a, [])
∀ x, a. perm(x, y0 + a ') ↔ a ∈ x ∧ perm(x -1l a, y0) ⊦ perm(x, (a0 + y0) + a ') ↔ a ∈ x ∧ perm(x -1l a, a0 + y0)
Used simplifier rules: list freely generated by [], +;;