def append_child ( self, object, child ):
        """ Appends a child to the object's children.
        """
        if isinstance( child, Subgraph ):
            object.subgraphs.append( child )

        elif isinstance( child, Cluster ):
            object.clusters.append( child )

        elif isinstance( child, Node ):
            object.nodes.append( child )

        elif isinstance( child, Edge ):
            object.edges.append( child )

        else:
            pass
Appends a child to the object's children.