x + y = z + a ', b ∈ x, a ∈ x ∨ a ∈ y ⊦ a = b, b ∈ z
Used simplifier rules: in-15: ⊦ x + a ' = y → a ∈ y;%a: ⊦ (x + y) + z = x + y + z;%in-append: ⊦ a ∈ x + y ↔ ¬ ¬(a ∈ x ∨ a ∈ y);