| ∀ n. n < # buf0 + # buf1 → (buf0 + buf1)[n] = empty ⊦ (n < # buf0 → buf0[n] = empty) ∧ (n0 < # buf1 → buf1[n0] = empty) |
n < # buf0, ∀ n. n < # buf0 + # buf1 → (buf0 + buf1)[n] = empty ⊦ buf0[n] = empty
n0 < # buf1, ∀ n. n < # buf0 + # buf1 → (buf0 + buf1)[n] = empty ⊦ buf1[n0] = empty