Comment by twanvl

> I'd be interested to see what a sufficiently strong QuickCheck specification of this problem would look like.

I would write something like this in haskell:

    spec :: [Integer] -> Property
    spec xs =
       length xs <= 2  ==>  fun (intercalate "," (map show xs)) == sum xs

This captures the three requirements, but not the implicit fourth requirement that the function throws an exception for other inputs.