code: def pack(self, value=None):
          
          if value is not None:
              return value.pack()
  
          # Set oxm_field_and_mask instance attribute
          # 1. Move field integer one bit to the left
          try:
              field_int = self._get_oxm_field_int()
          except ValueError as exception:
              raise PackException(exception)
          field_bits = field_int << 1
          # 2. hasmask bit
          hasmask_bit = self.oxm_hasmask & 1
          # 3. Add hasmask bit to field value
          self.oxm_field_and_mask = field_bits + hasmask_bit
  
          self._update_length()
          return super().pack(value)
docs: Join oxm_hasmask bit and 7-bit oxm_field.