⊦ x = y ↔ # x = # y ∧ (∀ n. n < # x → x[n] = y[n])
#(x0 + z) = #(x1 + x2), # x1 + n < # x0, # x2 < n, ∀ n. n < #(x0 + z) → (x0 + z)[n] = (x1 + x2)[n] ⊦ x0 = []
Used terms: x1 + x2, x0 + z