code: def twig(self):
          
          return "@".join([getattr(self, k) for k in _meta_fields_twig if getattr(self, k) is not None])
docs: The twig of a Parameter is a single string with the individual
          :meth:`meta` tags separated by '@' symbols.  This twig gives
          a single string which can point back to this Parameter.
  
          see also :meth:`uniquetwig`
  
          :return: twig (full) of this Parameter