code: def _tls_mac_add(alg, c, write_seq_num):
      
      write_seq_num = struct.pack("!Q", write_seq_num)
      h = alg.digest(write_seq_num + bytes(c))
      c.data += h
      c.len += alg.hash_len
docs: Compute the MAC using provided MAC alg instance over TLSCiphertext c using
      current write sequence number write_seq_num. Computed MAC is then appended
      to c.data and c.len is updated to reflect that change. It is the
      caller responsibility to increment the sequence number after the operation.
      The function has no return value.