??? vague thought about for example grothendieck topologies on comm monoid N^2 ... most as constraining some particular among (1,0) and (0,1) to act invertibly ... but then the interesting one, which i played around with as maybe being sort of like "_either_ of (1,0) or (0,1) must act invertibly ..." .... ??? .... probably intended as joke, more or less ... but seems partway true, in peculiar/annoying (? ...) way ... separated presheaf here as "on each pair of elements, either (1,0) or (0,1) acts injectively ..." ... ???? ...... but actual sheaf condition here as trickier ??? ... ???? .....