T15116a.hs:6:21: error: [GHC-88634]
    • Data constructor ‘MkB’ cannot be used here
        (it is defined and used in the same recursive group)
    • In the first argument of ‘Proxy’, namely ‘'MkB’
      In the type ‘(Proxy 'MkB)’
      In the definition of data constructor ‘MkB’