namespace Stack
{
    const int MAX_SIZE = 200;
    struct Rep;
    typedef Rep& stack;

    stack create();
    void destroy(stack s);

    void push(stack s, int i);
    int pop(stack s);
}

struct Stack::Rep
{
    int ar[200];
    int top;
};

"expected a identifier" and "initial value of reference to non-const must be an lvalue" :

Stack::stack Stack::create()
{
   Stack::stack = new Rep();
}

"reference variable "s" requires an initializer"

Stack::stack Stack::create()
{
   Stack::stack s;
}