Revive gitlab MR 297 by @mattwala

This revives (temporarily merged and then !309 (merged) reverted changes) from !297 (merged).

@mattwala, sorry for making a huge smouldering mess of this. As I said, I think this breaks sumpy. I'll start a test balloon MR there to check.

WIP until sumpy!108 (closed) passes unmodified.

Closes #152 (closed)