Double.hs:11:20: error:
    • Couldn't match expected type ‘MapAsSet.Map k0 a0’
                  with actual type ‘Map.Map Int [Char]’
      NB: ‘Map.Map’ is defined at Map.hsig:15:1-12
          ‘MapAsSet.Map’ is defined at MapAsSet.hsig:7:1-12
    • In the first argument of ‘keysSet’, namely ‘x’
      In the first argument of ‘print’, namely ‘(keysSet x)’
      In a stmt of a 'do' block: print (keysSet x)