T3169.hs:13:13:
    Couldn't match type `elt' with `Map b elt'
      `elt' is a rigid type variable bound by
            the type signature for
              lookup :: (a, b) -> Map (a, b) elt -> Maybe elt
            at T3169.hs:12:3
    Expected type: Maybe (Map b elt)
      Actual type: Maybe elt
    In the return type of a call of `lookup'
    In the expression: lookup a m :: Maybe (Map b elt)