@Override
public void collapse() {
assert collapsible : "Calling collapse on non-collapsible field set";
if (collapsible) {
if (fireCancellableEvent(new BeforeCollapseEvent())) {
this.collapsed = true;
appearance.onCollapse(getElement(), true);
getCollapseButton().changeStyle(ToolButton.DOWN);
fireEvent(new CollapseEvent());
}