Sounds like this make a sandwich exercise is rather good for understanding the issues with gathering software requirements; it confronts the students with creeping requirements, before they even know how to code.
Coding should probably be taught with a background of requirements that are precisely given in terms of the abstractions that are being used in the code. So that is to say, the problem itself doesn't have to be something abstract (like a number-theoretic computation); it can be connected to something in the real world. But the representational mapping from that domain to the computer should be established by the teacher, so there is no irrelevant fumbling about the requirements.
In the case of the sandwich, we should provide a procedure which somehow takes the slices of bread out of the bag. Similarly, we provide all other necessary procedures, and only those procedures are to be used. Then we eliminate questions about the detailed requirements about what those procedures themselves do, like can the bag be ripped, or does the removal have to be tidy.
A good domain for teaching coding might be game playing (cards, etc). People understand that the objects used in games are abstract, and that certain details do not matter to the game, like the exact shape of a rook in chess or its precise placement within a square. It could just be the letter "R" in an 8x8 grid of characters (with "r" being the opposite color).
Symbolic games are basically a form of mathematics that is instantiated in the real world whose abstractions people readily understand. Moreover, they have a point (to have fun), so the "why are we doing this" question is settled.