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