public interface NestingInterface{
  public boolean isEmpty();
  //post: Returns true iff this is empty();

  public Object getRootItem() throws NestingException;
  //post: Returns value associated with the root if this is not
  //      empty; otherwise throws NestingException.

  public void setRootItem(Object newItem);
  //post: Sets the value associated with the root to be newItem
  //      if this is not empty; otherwise sets this to consist of a 
  //      root node only, with root item set to newItem

  public void makeEmpty();
  //post: this is empty.

}