public interface SimpleBinaryTree {
	public Position root();
	public Position parent(Position v);
	public Position leftChild(Position v);
	public Position rightChild(Position v);
	public boolean isInternal(Position v);
	public boolean isExternal(Position v);
	public boolean isRoot(Position v);
	public void expandExternal(Position v);
	public void removeAboveExternal(Position V);
	public Object replace(Position v, Object e);
}