How to use the moltemplate.nbody_graph_search.Dgraph.NULL function in moltemplate

def IsCyclic(self):
        """
        IsCyclic() returns True if the graph is cyclic (and connected).
        (An exception is raised on disconnected graphs.)
        This function quits early as soon as a cycle is found.

        """
        self.Reset()
        if (type(self.g) is Ugraph):
            is_cyclic = self._IsCyclicUgraph(0, Dgraph.NULL)
        else:
            is_cyclic = self._IsCyclic(0)
        if ((self.sv != self.g.nv) and (not is_cyclic)):
            raise(Disconnected(self.g, "Error(IsCyclic): "+
                               "The input graph is not connected."))
        return is_cyclic

def FindEdge(self, istart, istop):
        """
        A simple function looks up the edge id number
        corresponding to an edge connecting vertex istart to istop.
        If not present returns Dgraph.NULL.

        """
        iv = istart
        for je in self.neighbors[iv]:
            jv = self.edges[je].stop
            if jv == istop:
                return je
        return Dgraph.NULL

def IsCyclic(self):
        """
        IsCyclic() returns True if the graph is cyclic (and connected).
        (An exception is raised on disconnected graphs.)
        This function quits early as soon as a cycle is found.

        """
        self.Reset()
        if (type(self.g) is Ugraph):
            is_cyclic = self._IsCyclicUgraph(0, Dgraph.NULL)
        else:
            is_cyclic = self._IsCyclic(0)
        if ((self.sv != self.g.nv) and (not is_cyclic)):
            raise(Disconnected(self.g, "Error(IsCyclic): " +
                               "The input graph is not connected."))
        return is_cyclic

def Reset(self):
        self.sv = 0
        self.se = 0
        for iv in range(0, self.g.nv):
            self.vvisited[iv] = False
            self.vorder[iv]   = Dgraph.NULL
        for ie in range(0, self.g.ne):
            self.evisited[ie] = False
            self.eorder[ie]   = Dgraph.NULL

def FindEdge(self, istart, istop):
        """
        A simple function looks up the edge id number
        corresponding to an edge connecting vertex istart to istop.
        If not present returns Dgraph.NULL.

        """
        iv = istart
        for je in self.neighbors[iv]:
            jv = self.edges[je].stop
            if jv == istop:
                return je
        return Dgraph.NULL

#      AND vertex Jv with jv
                        self.ie_to_Ie[self.se] = Je
                        self.se += 1
                        self.eoccupiedG[Je] = True
                        self.iv_to_Iv[self.sv] = Jv
                        self.sv += 1
                        self.voccupiedG[Jv] = True
                        # Then continue the recursion
                        for match in self.Match():
                            yield match
                        self.voccupiedG[Jv] = False
                        self.sv -= 1
                        self.iv_to_Iv[self.sv] = Dgraph.NULL
                        self.eoccupiedG[Je] = False
                        self.se -= 1
                        self.ie_to_Ie[self.se] = Dgraph.NULL

assert(not self.eoccupiedG[Je])
                        # Match both edge Je with je
                        #      AND vertex Jv with jv
                        self.ie_to_Ie[self.se] = Je
                        self.se += 1
                        self.eoccupiedG[Je] = True
                        self.iv_to_Iv[self.sv] = Jv
                        self.sv += 1
                        self.voccupiedG[Jv] = True
                        # Then continue the recursion
                        for match in self.Match():
                            yield match
                        self.voccupiedG[Jv] = False
                        self.sv -= 1
                        self.iv_to_Iv[self.sv] = Dgraph.NULL
                        self.eoccupiedG[Je] = False
                        self.se -= 1
                        self.ie_to_Ie[self.se] = Dgraph.NULL

def Reset(self):
        self.sv = 0
        self.se = 0
        for iv in range(0, self.g.nv):
            self.vvisited[iv] = False
            self.vorder[iv] = Dgraph.NULL
        for ie in range(0, self.g.ne):
            self.evisited[ie] = False
            self.eorder[ie] = Dgraph.NULL