docs: def tree_to_dot(tree:BubbleTree, dotfile:str=None, render:bool=False):
      """Write in dotfile a graph equivalent to those depicted in bubble file
  
      See http://graphviz.readthedocs.io/en/latest/examples.html#cluster-py
      for graphviz API
  
      """
      graph = tree_to_graph(tree)
      path = None
      if dotfile:  # first save the dot file.
          path = graph.save(dotfile)
      if render:  # secondly, show it.
          # As the dot file is known by the Graph object,
          # it will be placed around the dot file.
          graph.view()
      return path