| ⊦ ∃ a0, x, y0, z0. a ' + y + a ' + z = x + a0 ' + y0 + a0 ' + z0 |
Used simplifier rules: Nil: ⊦ [] + x = x; from specification list%a: ⊦ (x + y) + z = x + y + z; from specification list%app-08: ⊦ a ' + x = a ' + y ↔ x = y; from specification list Used terms: a, [], y, z