def huffman_conv2str(cls, bit_str, bit_len):
        # type: (int, int) -> str
        """ huffman_conv2str converts a bitstring of bit_len bitlength into a
        binary string. It DOES NOT compress/decompress the bitstring!

        @param int bit_str: the bitstring to convert.
        @param int bit_len: the bitlength of bit_str.
        @return str: the converted bitstring as a bytestring.
        @raise AssertionError
        """
        assert(bit_str >= 0)
        assert(bit_len >= 0)

        byte_len = bit_len // 8
        rem_bit = bit_len % 8
        if rem_bit != 0:
            bit_str <<= 8 - rem_bit
            byte_len += 1

        # As usual the list/join tricks is a performance trick to build
        # efficiently a Python string
        s = []  # type: List[str]
        i = 0
        while i < byte_len:
            s.insert(0, chb((bit_str >> (i * 8)) & 0xFF))
            i += 1
        return b''.join(s)