public OpenConfigAssignmentHandler addConfig(OpenConfigConfigOfAssignmentHandler config) {
        modelObject.config(config.getModelObject());
        return this;
    }
@Test
    public void testAddConfig() {
        // test Handler
        OpenConfigAssignmentHandler assignment = new OpenConfigAssignmentHandler(2, parent);

        // call addConfig
        OpenConfigConfigOfAssignmentHandler configOfAssignment =
            new OpenConfigConfigOfAssignmentHandler(assignment);

        // expected ModelObject
        DefaultAssignment modelObject = new DefaultAssignment();
        modelObject.index(2);
        DefaultConfig config = new DefaultConfig();
        modelObject.config(config);

        assertEquals("[NG]addConfig:ModelObject(Config added) is not an expected one.\n",
                     modelObject, assignment.getModelObject());
    }