docs: def mend(self, length):
          """Cut all branches from this node to its children and adopt
          all nodes at certain level."""
          if length == 0: raise Exception("Can't mend the root !")
          if length == 1: return
          self.children = OrderedDict((node.name, node) for node in self.get_level(length))
          for child in self.children.values(): child.parent = self