So I think that you could absolutely patch a system that's quite similar to `Idempotency-Key` into `ETag`, but you might be pushing the original concept far eno

So I think that you could absolutely patch a system that's quite similar to `Idempotency-Key` into `ETag`, but you might be pushing the original concept far enough base to the point where you're not gaining much by doing so. Using `If-Match` is essentially indicating that you want to make a request conditionally as long as the server's state matches a nonce that you're holding. Presumably that nonce was handed to you already by the server on an initial request that you already executed. You can hand Stripe's API an `Idempotency-Key` on the first request that you make against it. Furthermore, you'd never say that the first request made in this way was meant to be conditional, even if subsequent retries (after an initial failure) might be. I think it wouldn't be a problem in practice to just retrofit `ETag` to do the same thing, but doing so is (arguably) semantically wrong, and I think there's something to be said for the clarity that just using your own header with an obvious name like `Idempotency-Key`. I'm open to be persuaded though :)