code: def common_twig(self):
          
          return "@".join([getattr(self, k) for k in _meta_fields_twig if self.meta.get(k) is not None])
docs: The twig that is common between all items in this ParameterSet.
          This twig gives a single string which can point back to this ParameterSet
          (but may include other entries as well)
  
          see also :meth:`uniquetwig`
  
          :return: twig (full) of this Parameter