Return (and create if needed) the internal inventory field, a SetField
        used to track all dynamic versions used on a specific instance.

def _inventory(self):
        """
        Return (and create if needed) the internal inventory field, a SetField
        used to track all dynamic versions used on a specific instance.
        """
        if self.dynamic_version_of:
            return self.dynamic_version_of._inventory

        if not hasattr(self, '_inventory_field'):
            self._inventory_field = limpyd_fields.SetField()
            self._inventory_field._attach_to_model(self._model)
            self._inventory_field._attach_to_instance(self._instance)
            self._inventory_field.lockable = True
            self._inventory.name = self.name

        return self._inventory_field