T4179.hs:26:16:
    Couldn't match type ‘A2 (x (A2 (FCon x) -> A3 (FCon x)))’
                   with ‘A2 (FCon x)’
    NB: ‘A2’ is a type function, and may not be injective
    Expected type: x (A2 (FCon x) -> A3 (FCon x))
                   -> A2 (FCon x) -> A3 (FCon x)
      Actual type: x (A2 (FCon x) -> A3 (FCon x))
                   -> A2 (x (A2 (FCon x) -> A3 (FCon x)))
                   -> A3 (x (A2 (FCon x) -> A3 (FCon x)))
    In the first argument of ‘foldDoC’, namely ‘op’
    In the expression: foldDoC op
    Relevant bindings include
      fCon :: Con x -> A2 (FCon x) -> A3 (FCon x)
        (bound at T4179.hs:26:1)