| ∀ a, x0. x1 \ (a ' + x0) = x1 \ x0 ∨ a ∈ x1 ⊦ (a0 ' + x1) \ (a ' + x0) = (a0 ' + x1) \ x0, a = a0, a ∈ x1 |
⊦ ¬ a ∈ y → (a ' + x) \ y = a + (x \ y)
¬ ¬ a0 ∈ a ' + x0, ∀ a, x0. x1 \ (a ' + x0) = x1 \ x0 ∨ a ∈ x1 ⊦ (a0 ' + x1) \ (a ' + x0) = (a0 ' + x1) \ x0, a = a0, a ∈ x1
¬ a0 ∈ a ' + x0, ∀ a, x0. x1 \ (a ' + x0) = x1 \ x0 ∨ a ∈ x1 ⊦ a0 + (x1 \ (a ' + x0)) = (a0 ' + x1) \ x0, a = a0, a ∈ x1
Used terms: a ' + x0, x1, a0