T3169.hs:13:22:
    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: Map a (Map b elt)
      Actual type: Map (a, b) elt
    Relevant bindings include
      m :: Map (a, b) elt (bound at T3169.hs:12:17)
      b :: b (bound at T3169.hs:12:13)
      lookup :: (a, b) -> Map (a, b) elt -> Maybe elt
        (bound at T3169.hs:12:3)
    In the second argument of ‘lookup’, namely ‘m’
    In the expression: lookup a m :: Maybe (Map b elt)