My solution passes all the test cases but gives the error:

does not cheat
     scanl is not used

All of my answers were one line calls to scanl so I'm not sure what it's upset about. It possible to fix this one? It's a pretty fun problem.

Could really do with more rigorous testing.

I'd allow one extra character for at least some variability in possible solutions, and they would still require firm grasp of the concept