// Textbook fragment 13.04
  /** Setup method that is called prior to the DFS execution. */
  protected void setup() {}
  /** Initializes result (called first, once per vertex visited). */
  protected void initResult() {}
  /** Called when we encounter a vertex (v). */
  protected void startVisit(Vertex<V> v) {}
  /** Called after we finish the visit for a vertex (v). */
  protected void finishVisit(Vertex<V> v) {}
  /** Called when we traverse a discovery edge (e) from a vertex (from). */
  protected void traverseDiscovery(Edge<E> e, Vertex<V> from) {}
  /** Called when we traverse a back edge (e) from a vertex (from). */
  protected void traverseBack(Edge<E> e, Vertex<V> from) {}
  /** Determines whether the traversal is done early. */
  protected boolean isDone() { return false; /* default value */ }
  /** Returns a result of a visit (if needed). */
  protected R result() { return null; /* default value */ }
  /** Returns the final result of the DFS execute method. */
  protected R finalResult(R r) { return r; /* default value */ }