docs: def build_from_packet(pkt):
          '''
          Build and return a new OpenflowMatch object based on the
          packet object passed as a parameter.
          '''
          m = OpenflowMatch()
          for mf,pkttuple in OpenflowMatch._match_field_to_packet.items():
              for pktcls,field in pkttuple:
                  if pkt.has_header(pktcls):
                      setattr(m, mf, getattr(pkt[pktcls], field))
                      continue
          return m