Hello,

> data LSet t where
>     Nil :: LSet Nil
>     Ins :: (Member a t b
>           , If b t (a ::: t) r)
>           => L a -> LSet t -> LSet r
>
Try replacing both original occurrences of r, i.e. (untested)

    Ins :: (Member a t b, If b t (a ::: t) (LSet r)) => L a -> LSet t -> LSet r

-Jeff


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