class STACK a where {
  type StackRepr a elem;
  emptyStack :: a -> StackRepr a elem;
  top        :: a -> StackRepr a elem -> elem;
  pop        :: a -> StackRepr a elem -> StackRepr a elem;
  push       :: a -> elem -> StackRepr a elem -> StackRepr a elem;
}

data Stack = Stack;

instance STACK Stack where {
  type StackRepr Stack elem = List elem;
  emptyStack _      = Nil;
  top _ (Cons x xs) = x;
  pop _ (Cons x xs) = xs;
  push _ x xs       = Cons x xs;
}