| ∀ a. x0 \ a ' = x0 ∨ a ∈ x0 ⊦ (a0 + x0) \ a ' = a0 + x0, a ∈ a0 + x0 |
∀ a. x0 \ a ' = x0 ∨ a ∈ x0 ⊦ (a0 ' + x0) \ a ' = a0 ' + x0, a = a0, a ∈ x0
Used simplifier rules: a: ⊦ (x + y) + z = x + y + z; from specification list%in-06: ⊦ ¬ a ∈ b ' + x ↔ a ≠ b ∧ ¬ a ∈ x; from specification list%c: ⊦ a + x = a ' + x; from specification list