⊦ ∃ y, a0. a ' = y + a0 '
Used simplifier rules: c: ⊦ a + x = a ' + x;%app: ⊦ x + [] = x;%a: ⊦ (x + y) + z = x + y + z;%e-02: ⊦ x + a ' ≠ [];