mc21.hs:12:26: error:
    Couldn't match type ‘a’ with ‘[a]’
    ‘a’ is a rigid type variable bound by
    a type expected by the context:
      forall a. [a] -> [[a]]
    at mc21.hs:11:9
    Expected type: [a] -> [[a]]
      Actual type: [[a]] -> [[a]]
    In the expression: take 5
    In a stmt of a monad comprehension: then group using take 5