class SidebarLayout extends CssLayout {

    public void select(SidebarSheet sheet) {
        if (!sidebarSheets.contains(sheet)) {
            throw new IllegalArgumentException("The sheet " + sheet + " is not part of this SidebarLayout");
        }
        if (selected == sheet) {
            return;
        }
        Component content = sheet.getContent();
        if (content.getParent() == null) {
            contentArea.addComponent(content);
        } else if (content.getParent() != contentArea) {
            throw new IllegalStateException("Content of the selected sidebar sheet already has a parent!");
        }

        if (selected != null) {
            selected.unselect();
        }
        this.selected = sheet;
        sheet.select();
    }

    public  SidebarLayout();

}