ElemsRep.fsx

type Meta = interface end

type U<'ty>() =
    class
        interface Meta
    end

type K<'ty, 'x>(elem: 'x) =
    interface Meta
    member x.Elem = elem

type Id<'ty>(elem: 'ty) =
    interface Meta
    member x.Elem = elem

type Sum<'ty,'a,'b when 'a :> Meta and 'b :> Meta>(elem: Choice<'a,'b>) =
    interface Meta
    member x.Elem = elem

type Prod<'ty, 'a, 'b when 'a :> Meta and 'b :> Meta>(e1: 'a, e2: 'b) =
    interface Meta
    member x.Elem = e1, e2
    member x.E1 = e1
    member x.E2 = e2

type Elems =
    | Cons of int * Elems
    | Val of int
    | Nil

type ElemsRep =
    Sum<Elems,
        Sum<Elems,
            Prod<Elems, K<Elems, int>, Id<Elems>>,
            K<Elems, int>>,
        U<Elems>>

Perhaps this is what was intended

type ElemsRep =
    Sum<Elems,
        Sum<Elems,
            Prod<Elems, K<Elems, int>, Prod<Elems, Id<Elems>, U<Elems>>>,
            Sum<Elems,
                Prod<Elems, K<Elems, int>, U<Elems>>,
                U<Elems>>>,
        U<Elems>>

Or this:

type ElemsRep =
    Prod<Elems,
            Sum<Elems,
                Prod<Elems, K<Elems, int>, Prod<Elems, Id<Elems>, U<Elems>>>,
                Sum<Elems,
                    Prod<Elems, K<Elems, int>, U<Elems>>,
                    U<Elems>>>,
            U<Elems>>