Hello,

> data LSet t where
>     Nil :: LSet Nil
>     --either add the new element or do nothing
>     Ins :: (Member a t b
>           , If b (LSet t) (LSet (a ::: t)) r)
>           => L a -> LSet t -> r
>
The constructor Ins needs to return an LSet. Maybe try replacing occurrences of r with (LSet r).

-Jeff

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