⊦ ¬ a ∈ x → x \ (a ' + x0) = x \ x0
∀ a. a ' \ x0 = a ' ∨ a ∈ x0 ⊦ a ' \ x0 = a ', a = a0, a ∈ x0, a0 ∈ a '
Used simplifier rules: ⊦ a ∈ b ' ↔ a = b; (s) Used terms: x0, a0, a '