my $twig =
            XML::Twig->new(
                start_tag_handlers => {
                        'UML:Class' => \&uml_class_start,
                        'UML:Package' => \&uml_package_start,
                },
                twig_handlers => {
                        # purge at the end of each class section
                        'UML:Class' => sub { $_[0]->purge },
                        'UML:Attribute' => \&uml_attribute,
                        'UML:Generalization' => \&uml_generalization,
                        'UML:TaggedValue' => \&uml_taggedvalue,
                },
            );