finding an unused collision layer?

[ownlyme]

i still wish we could just define collision layer, because it's not really possible to ensure compatibility between mods, especially when a mod alters tile collision layers.
or maybe we modders should agree on some conventions regarding collision masks.
For example using layer-11 only for sub-entities that just collide with the actual entity (So does the realistic reactors mod)
I'm asking because my new treefarm mod alters the collision masks of not(grass or dirt), and i'd actually like to add another one for not snow... but who knows how many mods i'm breaking with that.