[Scripting Question] Inverting Happiness Gain

So there's this held item that, among other things, causes a Pokemon to lose happiness whenever they would gain it (to support moves like Frustration). Problem is, I have no idea how to even approach implementing it.

Halp?

You should take a look at def changeHappiness in PokeBattle_Pokemon (v16.2).
I only have like 2 minutes but the code has an example with the soothe bell increasing a positive gain.
same thing.

...
I have no idea why I didn't consider the Soothe Bell, thanks!