docs: def run_dot(dot):
      """Converts a graph in DOT format into an IPython displayable object."""
      global impl
      if impl is None:
          impl = guess_impl()
      if impl == "dot":
          return run_dot_dot(dot)
      elif impl == "js":
          return run_dot_js(dot)
      else:
          raise ValueError("unknown implementation {}".format(impl))