final XmlSequenceGroup workbookContentModel = (XmlSequenceGroup) workbookElementDef.getContentModel();
final List<XmlContentModel> workbookNestedContent = workbookContentModel.getNestedContent();
assertEquals( 1, workbookNestedContent.size() );
final XmlContentModelReference shapesGroupRef = (XmlContentModelReference) workbookNestedContent.get( 0 );
final XmlGroupContentModel shapesGroup = (XmlGroupContentModel) shapesGroupRef.getContentModel();
final List<XmlContentModel> shapesGroupNestedContent = shapesGroup.getNestedContent();
assertEquals( 2, shapesGroupNestedContent.size() );
final XmlElementDefinition circleElementDef = (XmlElementDefinition) shapesGroupNestedContent.get( 0 );