[pierrec]

Jeskola Relativion does exactly what you describe. Relatively obscure because it's exclusively a Buzz plugin and it's not documented anywhere. I believe it comes with Buzz, it's mentioned in the changelog here: https://jeskola.net/buzz/beta/files/changelog.txt

That said, it's a fairly simple thing to develop, and I wouldn't be surprised if there are a bunch of other implementations.