[VLM]

The rule relies inductively on proving a nand (or nor or any one of them in the class) is a universal computer, then any other gate where you can hardwire an input to construct an inverter can be treated as that universal gate with some combination of the previously mentioned inverters slapped on inputs and/or output. I don't remember the details either, LOL, but that was the general gist of it, if you can make inverters out of an alternative then you can slap enough inverters on it to make a NAND eventually.