∀ {b : ByteArray} (hb : 1 ≤ b.size)
  (h : ByteArray.utf8DecodeChar?.parseFirstByte b[0] = ByteArray.utf8DecodeChar?.FirstByte.done),
  b.utf8DecodeChar? 0 = ByteArray.utf8DecodeChar?.assemble₁ b[0] h