I built an RS232 cable from parts from RadioShack 25 years ago, with no soldering, just electrical tape. It’s surprisingly easy if you don’t need speed. Mine capped out at 1200 or 2400 baud. Was it good? No. Did it work? Absolutely.

https://en.wikipedia.org/wiki/Peano_axioms

In this case, they’re talking about being in the natural numbers club, and applying the “successor” (or “succ”) function.