code: def _get_substructure(self, treelist, pos):
          
          subtree = None
          if len(pos) > 1:
              subtree = self._get_substructure(treelist[pos[0]][1], pos[1:])
          else:
              try:
                  subtree = treelist[pos[0]]
              except (IndexError, TypeError):
                  pass
          return subtree
docs: recursive helper to look up node-tuple for `pos` in `treelist`