T3169.hs:13:22: error:
    • Occurs check: cannot construct the infinite type: elt ~ Map b elt
      Expected type: Map a (Map b elt)
        Actual type: Map (a, b) elt
    • In the second argument of ‘lookup’, namely ‘m’
      In the expression: lookup a m :: Maybe (Map b elt)
      In the expression:
        case lookup a m :: Maybe (Map b elt) of {
          Just (m2 :: Map b elt) -> lookup b m2 :: Maybe 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)