// Empty stacks.
      // 
      // For purpose statements and algebraic specification, see Stack.java.
      
      public class EmptyStack<E> extends StackBase<E> {
      
          public EmptyStack () { }
      
          public Stack<E> push (E x) {
              return new Push<E> (this, x);
          }
      
          public boolean  isEmpty () {
              return true;
          }
      
          public E top () {
              String msg1
                  = "attempted to compute the top of an empty StackInt";
              throw new RuntimeException (msg1);
          }
      
          public Stack<E> pop () {
              return this;                   // FIXME: this is evil
          }
      
          public int size () {
              return 0;
          }
      }
    

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