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

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