docs: def is_active_trail(self, start, end, observed=None):
          """
          Returns True if there is any active trail between start and end node
          Parameters
          ----------
          start : Graph Node
          end : Graph Node
          observed : List of nodes (optional)
              If given the active trail would be computed assuming these nodes to be observed.
          additional_observed : List of nodes (optional)
              If given the active trail would be computed assuming these nodes to be observed along with
              the nodes marked as observed in the model.
          Examples
          --------
          >>> from pgmpy.base import DAG
          >>> student = DAG()
          >>> student.add_nodes_from(['diff', 'intel', 'grades', 'letter', 'sat'])
          >>> student.add_edges_from([('diff', 'grades'), ('intel', 'grades'), ('grades', 'letter'),
          ...                         ('intel', 'sat')])
          >>> student.is_active_trail('diff', 'intel')
          False
          >>> student.is_active_trail('grades', 'sat')
          True
          """
          if end in self.active_trail_nodes(start, observed)[start]:
              return True
          else:
              return False