⊦ (∃ z. (a0 ' + x0) + z = a ') ↔ a0 ' + x0 = a '
Used simplifier rules: ⊦ a ' + x ≠ []; (s)%a: ⊦ (x + y) + z = x + y + z; from specification list% ⊦ a + x = a ' + x; (s)