mc24.hs:10:31:
    Couldn't match type ‘[a0]’ with ‘[a] -> m [a]’
    Expected type: (a -> Integer) -> [a] -> m [a]
      Actual type: [a0] -> [a0]
    Possible cause: ‘take’ is applied to too many arguments
    In the expression: take 2
    In a stmt of a monad comprehension: then group by x using take 2
    Relevant bindings include foo :: m Int (bound at mc24.hs:8:1)