* viewed, it becomes immutable
*/
static class Ref<T> extends Forest<T> {
* viewed, it becomes immutable
*/
static class Ref<T> extends Forest<T> {
public boolean ambiguous() {
if (hp.size()==0) return false;
if (hp.size()==1) return hp.iterator().next().ambiguous();
public boolean ambiguous() {
if (hp.size()==0) return false;
if (hp.size()==1) return hp.iterator().next().ambiguous();