def to_string(self):
        """Return string representation of graph in DOT language.

        @return: graph and subelements
        @rtype: `str`
        """


        graph = list()

        if self.obj_dict.get('strict', None) is not None:

            if (self == self.get_parent_graph() and
                    self.obj_dict['strict']):

                graph.append('strict ')

        graph_type = self.obj_dict['type']
        if (graph_type == 'subgraph' and
                not self.obj_dict.get('show_keyword', True)):
            graph_type = ''
        s = '{type} {name} {{\n'.format(
            type=graph_type,
            name=self.obj_dict['name'])
        graph.append(s)

        for attr in sorted(self.obj_dict['attributes']):

            if self.obj_dict['attributes'].get(attr, None) is not None:

                val = self.obj_dict['attributes'].get(attr)
                if val == '':
                    val = '""'
                if val is not None:
                    graph.append('%s=%s' %
                                 (attr, quote_if_necessary(val)))
                else:
                    graph.append( attr )

                graph.append( ';\n' )


        edges_done = set()

        edge_obj_dicts = list()
        for k in self.obj_dict['edges']:
            edge_obj_dicts.extend(self.obj_dict['edges'][k])

        if edge_obj_dicts:
            edge_src_set, edge_dst_set = list(zip(
                *[obj['points'] for obj in edge_obj_dicts]))
            edge_src_set, edge_dst_set = set(edge_src_set), set(edge_dst_set)
        else:
            edge_src_set, edge_dst_set = set(), set()

        node_obj_dicts = list()
        for k in self.obj_dict['nodes']:
            node_obj_dicts.extend(self.obj_dict['nodes'][k])

        sgraph_obj_dicts = list()
        for k in self.obj_dict['subgraphs']:
            sgraph_obj_dicts.extend(self.obj_dict['subgraphs'][k])


        obj_list = [(obj['sequence'], obj)
                    for obj in (edge_obj_dicts +
                                node_obj_dicts + sgraph_obj_dicts) ]
        obj_list.sort(key=lambda x: x[0])

        for idx, obj in obj_list:

            if obj['type'] == 'node':

                node = Node(obj_dict=obj)

                if self.obj_dict.get('suppress_disconnected', False):

                    if (node.get_name() not in edge_src_set and
                        node.get_name() not in edge_dst_set):

                        continue

                graph.append( node.to_string()+'\n' )

            elif obj['type'] == 'edge':

                edge = Edge(obj_dict=obj)

                if (self.obj_dict.get('simplify', False) and
                        edge in edges_done):
                    continue

                graph.append( edge.to_string() + '\n' )
                edges_done.add(edge)

            else:

                sgraph = Subgraph(obj_dict=obj)

                graph.append( sgraph.to_string()+'\n' )

        graph.append( '}\n' )

        return ''.join(graph)
Return string representation of graph in DOT language.

        @return: graph and subelements
        @rtype: `str`