code: def xyz(self):
          
          if not self.children:
              pos = np.expand_dims(self._pos, axis=0)
          else:
              arr = np.fromiter(itertools.chain.from_iterable(
                  particle.pos for particle in self.particles()), dtype=float)
              pos = arr.reshape((-1, 3))
          return pos
docs: Return all particle coordinates in this compound.
  
          Returns
          -------
          pos : np.ndarray, shape=(n, 3), dtype=float
              Array with the positions of all particles.