type IStack
{
    void push (Any elt);
    Any pop();
    bool empty();
    IStack copy();
}

class ListStack() : IStack impl IStack {
    List stack = new MTList();

    void push (Any elt) {
       stack = new ConsList(car = elt, cdr = stack);
    }

    Any pop() {
       if stack.empty() {
         error("The stack is empty!");
       } else {
         Any x = stack.first();
         stack = stack.drop(1);
         return x;
       };
    }

    bool empty() {
      return stack.empty();
    }

    IStack copy() {
      List tocopy = stack.reverse();
      IStack newStack = new ListStack();
      while(!tocopy.empty()) {
        newStack.push(tocopy.first());
        tocopy = tocopy.drop(1);
      };
      return newStack;
    }

    export IStack : push, pop, empty, copy;
}

type ICountedStack <: IStack
{
    int numElements();
}

mixin addCount() : ICountedStack at IStack impl ICountedStack
{
    int numElts = 0;

    super();

    void countedPush(Any elt)
    {
        numElts = numElts + 1;
        push(elt);
    }

    Any countedPop()
    {
        Any x = pop();
        numElts = numElts - 1;
        return x;
    }

    int getSize()
    {
        numElts;
    }

    export ICountedStack :
        countedPush as push,
        countedPop as pop,
        empty,
        copy, // but this isn't a countable stack!
        getSize as numElements;
}

subclass CountedListStack = addCount(ListStack);