⊦ ⌜x + y⌝ = ⌜x⌝ + ⌜y⌝
⊦ ⌜x1⌝ + ⌜x + y⌝ = ⌜x2⌝ + ⌜x0 + z⌝ ↔ x1 + x + y = x2 + x0 + z
Used terms: z, x0