How to use the pygraphviz.graphviz.agfstnode function in pygraphviz

def in_edges_iter(self, nbunch=None, keys=False):
        """Return iterator over out edges in the graph.

        If the optional nbunch (container of nodes) only out edges
        adjacent to nodes in nbunch will be returned.

        Note: modifying the graph structure while iterating over
        edges may produce unpredictable results.  Use in_edges()
        as an alternative.
        """
        if nbunch is None:   # all nodes
            nh = gv.agfstnode(self.handle)
            while nh is not None:
                eh = gv.agfstin(self.handle, nh)
                while eh is not None:
                    e = Edge(self, eh=eh)
                    if keys:
                        yield (e[0], e[1], e.name)
                    else:
                        yield e
                    try:
                        eh = gv.agnxtin(self.handle, eh)
                    except StopIteration:
                        break
                try:
                    nh = gv.agnxtnode(self.handle, nh)
                except StopIteration:
                    return

def nodes_iter(self):
        """Return an iterator over all the nodes in the graph.

        Note: modifying the graph structure while iterating over
        the nodes may produce unpredictable results.  Use nodes()
        as an alternative.
        """
        nh=gv.agfstnode(self.handle)
        while nh is not None:
            yield Node(self,nh=nh)
            nh=gv.agnxtnode(self.handle,nh)
        raise StopIteration

def out_edges_iter(self, nbunch=None, keys=False):
        """Return iterator over out edges in the graph.

        If the optional nbunch (container of nodes) only out edges
        adjacent to nodes in nbunch will be returned.

        Note: modifying the graph structure while iterating over
        edges may produce unpredictable results.  Use out_edges()
        as an alternative.
        """

        if nbunch is None:   # all nodes
            nh=gv.agfstnode(self.handle)
            while nh is not None:
                eh=gv.agfstout(self.handle,nh)
                while eh is not None:
                    e=Edge(self,eh=eh)
                    if keys:
                        yield (e[0],e[1],e.name)
                    else:
                        yield e
                    eh=gv.agnxtout(self.handle,eh)
                nh=gv.agnxtnode(self.handle,nh)
        elif nbunch in self: # if nbunch is a single node
            n=Node(self,nbunch)
            nh=n.handle
            eh=gv.agfstout(self.handle,nh)
            while eh is not None:
                e=Edge(self,eh=eh)

def in_edges_iter(self, nbunch=None, keys=False):
        """Return iterator over out edges in the graph.

        If the optional nbunch (container of nodes) only out edges
        adjacent to nodes in nbunch will be returned.

        Note: modifying the graph structure while iterating over
        edges may produce unpredictable results.  Use in_edges()
        as an alternative.
        """
        if nbunch is None:   # all nodes
            nh=gv.agfstnode(self.handle)
            while nh is not None:
                eh=gv.agfstin(self.handle,nh)
                while eh is not None:
                    e=Edge(self,eh=eh)
                    if keys:
                        yield (e[0],e[1],e.name)
                    else:
                        yield e
                    eh=gv.agnxtin(self.handle,eh)
                nh=gv.agnxtnode(self.handle,nh)
        elif nbunch in self: # if nbunch is a single node
            n=Node(self,nbunch)
            nh=n.handle
            eh=gv.agfstin(self.handle,nh)
            while eh is not None:
                e=Edge(self,eh=eh)

def nodes_iter(self):
        """Return an iterator over all the nodes in the graph.

        Note: modifying the graph structure while iterating over
        the nodes may produce unpredictable results.  Use nodes()
        as an alternative.
        """
        nh = gv.agfstnode(self.handle)
        while nh is not None:
            yield Node(self, nh=nh)
            try:
                nh = gv.agnxtnode(self.handle, nh)
            except StopIteration:
                return

def out_edges_iter(self, nbunch=None, keys=False):
        """Return iterator over out edges in the graph.

        If the optional nbunch (container of nodes) only out edges
        adjacent to nodes in nbunch will be returned.

        Note: modifying the graph structure while iterating over
        edges may produce unpredictable results.  Use out_edges()
        as an alternative.
        """

        if nbunch is None:   # all nodes
            nh = gv.agfstnode(self.handle)
            while nh is not None:
                eh = gv.agfstout(self.handle, nh)
                while eh is not None:
                    e = Edge(self, eh=eh)
                    if keys:
                        yield (e[0], e[1], e.name)
                    else:
                        yield e
                    try:
                        eh = gv.agnxtout(self.handle, eh)
                    except StopIteration:
                        break
                try:
                    nh = gv.agnxtnode(self.handle, nh)
                except StopIteration:
                    return