code: def return_xyz(self, labels=None):
          
          all_labels = self.return_label()
  
          if labels is None:
              labels = all_labels
  
          xyz = []
          for one_label in labels:
              idx = all_labels.index(one_label)
              xyz.append(self.chan[idx].xyz)
  
          return asarray(xyz)
docs: Returns the location in xy for some channels.
  
          Parameters
          ----------
          labels : list of str, optional
              the names of the channels.
  
          Returns
          -------
          numpy.ndarray
              a 3xn vector with the position of a channel.