def pack(self, value=None):
        """Join oxm_hasmask bit and 7-bit oxm_field."""
        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)
Join oxm_hasmask bit and 7-bit oxm_field.