-- Maybe is handled like the Maybe-of-Sum monoid
instance AdditiveGroup a => AdditiveGroup (Maybe a) where 
  zeroV = Nothing  Nothing ^+^ b' = b' 
  a' ^+^ Nothing      = a'
  Just a' ^+^ Just b' = Just (a' ^+^ b')
  negateV = fmap negateV

{- Alexey Khudyakov wrote: 

I looked through vector-space package and found lawless instance. Namely Maybe's AdditiveGroup instance  It's group so following relation is expected to hold. Otherwise it's not a group. 

> x ^+^ negateV x == zeroV 

Here is counterexample: 

> let x = Just 2 in x ^+^ negateV x == zeroV
False 

I think it's not possible to sensibly define group instance for Maybe a at all.I see that the problem here is in distinguishing 'Just zeroV' fromNothing. I could fix the Just + Just line to use Nothing instead of JustzeroV when a' ^+^ b' == zeroV, although doing so would require Eq a andhence lose some generality. Even so, the abstraction leak would probablyshow up elsewhere.Hm.
-}