Split function should work with Pairs and data classes

It would be great if the split function could automatically work with pairs and other data classes

Start by locating the split function and reading how it currently handles its inputs. Check for existing tests or examples involving Pair and data classes, then clarify the expected behavior for both before implementing. Done means split supports the agreed Pair and data-class cases with coverage for those cases.

Written by the indexing model from the issue text.