a ' + x = y + b ' + z, a ∈ y ∨ a ∈ b ' ∨ a ∈ z ⊦ y = [], b ∈ x
Used simplifier rules: in-14: ⊦ a ' + x = y → a ∈ y;%a: ⊦ (x + y) + z = x + y + z;%in-append: ⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y;