code: def groups(self):
          
          if len(self.groupings) == 1:
              return self.groupings[0].groups
          else:
              to_groupby = lzip(*(ping.grouper for ping in self.groupings))
              to_groupby = Index(to_groupby)
              return self.axis.groupby(to_groupby)
docs: dict {group name -> group labels}