T20666a.hs:11:10: warning: [GHC-36038] [-Wloopy-superclass-solve (in -Wdefault)]
    I am solving the constraint ‘Eq (F [a])’,
      arising from the superclasses of an instance declaration,
    in a way that might turn out to loop at runtime.
    Starting from GHC 9.10, this warning will turn into an error.
    See the user manual, § Undecidable instances and loopy superclasses.
    Suggested fix:
      Add the constraint ‘Eq (F [a])’ to the instance context,
      even though it seems logically implied by other constraints in the context.