@Override
	public String toString() {
		StringBuffer s = new StringBuffer();
		s.append("{");
		boolean first = true;
		for (TreeNode l : ingroup) {
			if (first) {
				first = false;
			} else {
				s.append(", ");
			}
			s.append(l.getLabel());
		}
		s.append("} | {");
		first = true;
		for (TreeNode l : outgroup) {
			if (first) {
				first = false;
			} else {
				s.append(", ");
			}
			s.append(l.getLabel());
		}
		s.append("}");
		return s.toString();
	}