class Stack[T]
{
   private var items=List[T]()
	
   def isEmpty=items.isEmpty

   def peek=items match{
      case List() => error("Stack empty")
      case head::rest => head
   }

   def pop=items match{
      case List() => error("Stack empty")
      case head::rest => items=rest; head
   }

   def push(value:T)=items=value+:items
}