| n < # buf0 + # buf1, ∀ n. n < # buf0 → buf0[n] = empty, ∀ n. n < # buf1 → buf1[n] = empty ⊦ (n < # buf0 ⊃ buf0[n];buf1[n - # buf0]) = empty |
n < # buf0 + # buf1, n < # buf0, ∀ n. n < # buf1 → buf1[n] = empty, ∀ n. n < # buf0 → buf0[n] = empty ⊦ buf0[n] = empty
n < # buf0 + # buf1, ∀ n. n < # buf1 → buf1[n] = empty, ∀ n. n < # buf0 → buf0[n] = empty ⊦ buf1[n - # buf0] = empty, n < # buf0