feature request

[:db/cas id attr old-val nil] should be legal

You could hack this maybe:

[:db/cas id attr old-val old-val] [:db/retract db attr old-val]

i.e. a no-op cas just as a precondition, then retract

however, the retract inherently won’t do anything if old-val is no longer asserted, so this is just picking between whether you want the transaction to succeed or fail--the end result by itself should be the same.

That's a pretty good hack for my use case. Thanks again, @U09R86PA4.

Can branching conditionals be done at all at this layer?

No.

Can you write a custom tx function for cloud? (I am not sure what that would look like personally. I've done some work for making it easier to write tx functions for on-prem)

Yes -and that's a reasonable fallback too.