code: public E pop() throws EmptyStackException
     {
       E answer = peek(); // Throws EmptyStackException if there's nothing there
       _elements.get().remove(_elements.get().size()-1);
       return answer;     
     }
docs: Pop and return the top element of the stack for this thread
  
  @return The previously top element of the stack for this thread
  @throws EmptyStackException is thrown if the stack for this thread is empty