GitLab

* Add ValueError

* Add tests for ValueError

* Add tests for ValueError

* Add ValueError

* Change to if/else

* Ammend iou_fn tests

* Move code excerpt

* Format tests
Co-authored-by: Philip Meier <github.pmeier@posteo.de>
Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>