However you can simply use a d-flipflop to do what you need.
The point of my exercise was to test a basic functionality. If you cannot simulate that, the feedback loops which arise naturally in more complex circuit designs won't work either. It's like the "fizzbuzz" of logic simulation.
If you have a flip-flop atom then you can do some meaningful stuff; IIRC the gate-level circuit simulator that the Nand To Tetris course uses can't handle this stuff, and you can still build a CPU. I guess it just depends at what level you want to think about the circuit.
Edit: maybe it should be called a "logic circuit" instead of a "digital circuit".
The point of my exercise was to test a basic functionality. If you cannot simulate that, the feedback loops which arise naturally in more complex circuit designs won't work either. It's like the "fizzbuzz" of logic simulation.