Forcing network to fault if two scalar integer values are different

[ArchRobison]

ONNX seems to be missing any kind of assertion facility. I'm looking for way to achieve a similar effect with what is in ONNX. For example, suppose I have two scalar integer tensors J and K that need to be equal, and I want an error to occur if they are not. Something that comes close is: gather([42], J-K), i.e. gather from a one-element tensor using the difference between J and K as the subscript. That almost works, except that gather allows negative subscripts, so gather([42],J-K) would succeed if J-K is 0 OR J-K is -1.

Suggestions? Shorter is better.