///////    ArbStack.java    ///////

//    generic stack uses-a generic list


class ArbStack
{  
   public ArbStack() { }               // default constructor
   public ArbStack(Object z)           // constructor
   {    al.putOn(z);   }
   public void push(Object z)
   {    al.putOn(z);   }
   public Object pop()
   {  if ( al.isEmpty() ) return(null);// pop failed
      Object tmp = al.first().content(); // popped item
      al.shorten(1);                   // stack popped
      return(tmp);
   }
   public boolean isEmpty()
   {  return al.isEmpty(); }

   private ArbList al = new ArbList();
}