code: func (b IPv4) SetFlagsFragmentOffset(flags uint8, offset uint16) {
  	v := (uint16(flags) << 13) | (offset >> 3)
  	binary.BigEndian.PutUint16(b[flagsFO:], v)
  }
docs: // SetFlagsFragmentOffset sets the "flags" and "fragment offset" fields of the
  // ipv4 header.