IPeCreateService peCreateService = Graphiti.getPeCreateService();
        ContainerShape containerShape =
            peCreateService.createContainerShape(targetDiagram, true);
        
        int xy[] = new int[] { width / 2, 0, width, height / 2, width / 2,
				height, 0, height / 2 };
		Polygon polygon = gaService.createPolygon(containerShape, xy);

	public GraphicsAlgorithm getSelectionBorder(PictogramElement pe) {
		IFeatureProvider featureProvider = getFeatureProvider();
		Object bo = featureProvider.getBusinessObjectForPictogramElement(pe);
		...
		} else if (bo instanceof Branch) {
			GraphicsAlgorithm ga = pe.getGraphicsAlgorithm();
			return ga;
		}

		return super.getSelectionBorder(pe);
	}