The current exercise expects `numerals :: Integer -> Maybe String`

which is pretty idiomatic for a function that wouldn’t work on all values of `Integer`

, but the tests only test for `Just`

return values.

It feels weird that the student can implement a partial function wrapped in `Just`

. Can we add Haskell-specific tests for numbers that cannot be rendered in roman numerals, like -1, 0 or 4000?