* @param erase if true, if there is an existing group with the same master,
* it will be erased. if not, the selected nodes will be added to the
* existing group.
*/
protected void collapse(boolean erase) {
GraphNode master = collapseMaster.getElementAtIndex(
masterViewer.getCombo().getSelectionIndex());
Collection<GraphNode> objects = collapseMaster.getObjects();
getEditor().collapse(master, objects, erase, null);
}