n ≤ # buf, ∀ n0. n0 < n → buf[n0] = empty ⊦ ∃ buf0. buf = mkempbuf(n) + buf0
n ≤ # buf, ∃ buf0. buf = mkempbuf(n) + buf0 ⊦ ∀ n0. n0 < n → buf[n0] = empty