code: def unique_id(self):
          
          chain = self.ampal_parent.ampal_parent.id
          residue = self.ampal_parent.id
          return chain, residue, self.id
docs: Creates a unique ID for the `Atom` based on its parents.
  
          Returns
          -------
          unique_id : (str, str, str)
              (polymer.id, residue.id, atom.id)