code: def add_group(self, L):
          
          if len(L) == 1:
              self.add(L[0])
          elif len(L) >= 2:
              x = PQ_node(P_shape)
              x.add_all(L)
              self.add(x)
docs: Add elements of L as descendants of the node.
          If there are several elements in L, group them in a P-node first