public interface InspectableTree extends InspectablePositionalContainer {
  // accessor methods
  /** return the root of the tree */
  public Position root();
  /** return the parent of a node */
  public Position parent(Position v);
  /** return the children of a node */
  public PositionIterator children(Position v);
  // query methods
  /** test whether a node is internal */
  public boolean isInternal(Position v);
  /** test whether a node is external */
  public boolean isExternal(Position v);
  /** test whether a node is the root of the tree */
  public boolean isRoot(Position v);
}