code: def join(self, *groupnames):
          
          return self._sum([self[k] for k in groupnames if k in self])
docs: Return an index group that contains atoms from all  *groupnames*.
  
          The method will silently ignore any groups that are not in the
          index.
  
          **Example**
  
          Always make a solvent group from water and ions, even if not
          all ions are present in all simulations::
  
             I['SOLVENT'] = I.join('SOL', 'NA+', 'K+', 'CL-')