when {
            b == 0x80 -> {
              // 10000000
              throw IOException("index == 0")
            }
            b and 0x80 == 0x80 -> {
              // 1NNNNNNN
              val index = readInt(b, PREFIX_7_BITS)
              readIndexedHeader(index - 1)
            }
            b == 0x40 -> {
              // 01000000