i + i = j0 ⊦ ∃ i0. i0 + i0 = j0 + 1 ∨ i0 + i0 = j0
i + i + 1 = j0 ⊦ ∃ i0. i0 + i0 = j0 + 1 ∨ i0 + i0 = j0