abstract class Stack{ 
  
  abstract public void push(Object el);
  abstract public void pop();
  abstract public Object top();
  abstract public boolean full();
  abstract public boolean empty();
  abstract public int size();

  public void toggleTop(){
    if (size() >= 2){
      Object topEl1 = top();  pop();
      Object topEl2 = top();  pop();
      push(topEl1); push(topEl2);
    }
  } // toggleTop

  public String toString(){
    return("Stack");
  }
} // end Stack