T3169.hs:13:22:
    Could not deduce (elt ~ Map b elt)
    from the context (Key a, Key b)
      bound by the instance declaration at T3169.hs:10:10-36
      ‛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)