be consumed

:param packet.Packet|None pkt: the packet containing this field.
  Probably unused.
:param str|(str, int, long) s: the string to append this field to.
  A tuple indicates that some bits were already generated by another
  bitfield-compatible field. This MUST be the case if "size" is not 8.
  The int is the number of bits already generated in the first byte of
  the str. The long is the value that was generated by the previous
  bitfield-compatible fields.
:param int val: the positive or null value to be added.
:return: str: s concatenated with the machine representation of this
  field.
:raises: AssertionError
r