In the instructions for each exercise it’s stated that we can use any components that are allowed in Exercise 1. Does that mean we can use subtractors to design preliminary exercise 1, despite there being written to use only basic gates?

- Those are typos. Exercises 1 to 4 should say “implement your design from
*Preliminary Exercise*1”. I hoped the incorrectly self-referential wording of Exercise 1 would have made that obvious, but nevertheless: Preliminary Exercise 1 asks your to design the circuit on paper, and Exercises 1 to 4 ask you to implement the design from Preliminary Exercise 1 in Logisim. Hence, if a component is not allowed in Preliminary Exercise 1, you cannot use it in Exercises 1 to 4. - Instruction #14 explicitly specifies the libraries from which you are allowed to use components. Subtractors are in the Arithmetic library, which is not allowed.
- Preliminary Exercise 1 explicitly mentions that you must use “exactly one instance of B2G(n - 1) […], at most one multiplexer, and a minimal number of basic logic gates.”

Therefore, for three separate reasons, you are most definitely *not* allowed to use subtractors.