| ∀ x, a. perm(x, y0 + a ') ↔ a ∈ x ∧ perm(x -1l a, y0) ⊦ a0 ∈ x ∧ perm(x -1l a0, y0 + a ') ↔ a ∈ x ∧ a0 ∈ x -1l a ∧ perm((x -1l a) -1l a0, y0) |
⊦ (x -1l a) -1l b = (x -1l b) -1l a
∀ x, a. perm(x, y0 + a ') ↔ a ∈ x ∧ perm(x -1l a, y0) ⊦ a0 ∈ x ∧ perm(x -1l a0, y0 + a ') ↔ a ∈ x ∧ a0 ∈ x -1l a ∧ perm((x -1l a0) -1l a, y0)
Used terms: a0, a, x