| a ∈ z, a ∈ z → (∃ y, z0. z = y + a ' + z0 ∧ ¬ a ∈ z0) ⊦ a0 ∈ z, ∃ b, y0, z0. y + a0 ' + z = y0 + b ' + z0 ∧ (b = a ∨ b = a0) ∧ ¬ a ∈ z0 ∧ ¬ a0 ∈ z0 |
⊦ a ∈ z0, a0 ∈ y0, a0 ∈ a ', a0 ∈ z0, ∃ b, y1, z1. y + a0 ' + y0 + a ' + z0 = y1 + b ' + z1 ∧ (b = a ∨ b = a0) ∧ ¬ a ∈ z1 ∧ ¬ a0 ∈ z1
Used simplifier rules: a: ⊦ (x + y) + z = x + y + z;%in-13: ⊦ x = y + a ' + z → a ∈ x;%in-append: ⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y;