Wingman produces silly arbitrary instance for single-constructor types

[isovector]

data CourseData = CD
  { cd_dur :: Dur
  , cd_end :: Time
  }

instance Arbitrary CourseData where
  arbitrary
    = let terminal = [(CD <$> arbitrary) <*> arbitrary]
      in
        sized
          $ (\ n
               -> case n <= 1 of
                    True -> oneof terminal
                    False -> oneof $ ([] <> terminal))