| ∃ a, x0, y, z. x = x0 + (a + (y + (a + z))) ⊦ ∃ a. a ∈ x ∧ #oc(a, x) ≠ 1 |
⊦ ∃ a0. (a0 ∈ x0 ∨ a0 = a ∨ a0 ∈ y ∨ a0 ∈ z) ∧ #oc(a0, x0) + #oc(a0, a ') + #oc(a0, y) + #oc(a0, a ') + #oc(a0, z) ≠ 1
Used simplifier rules: ocappend: ⊦ #oc(a, x + y) = #oc(a, x) + #oc(a, y);% ⊦ a ≠ b → (a ∈ b ' + x ↔ a ∈ x); (s)% ⊦ a ∈ b ' ↔ a = b; (s)% ⊦ a ∈ x + y ↔ a ∈ x ∨ a ∈ y; (s)%a: ⊦ (x + y) + z = x + y + z; from specification list% ⊦ a + x = a ' + x; (s)