anchor.diff:  
               | Fragment ->
                   None
  -//                match s with
  -//                    | "Positions" -> Some "gl_FragCoord"
  -//                    | _ -> None
  +
               | TessControl ->
                   match s with
                       | "Positions" -> Some "gl_Position"
  
positive.code:                             let unused = vs.outputs |> Map.filter (fun k v -> not <| Map.containsKey k tessUsed) |> Seq.map(fun (KeyValue(k,(_,v))) -> v) |> Set.ofSeq
                              let vs = removeOutputs unused vs
  
                              let vs = liftIntrinsics vs None ((if hasgs then Geometry TriangleStrip else Fragment) |> Some) 
                              
                              let! vsc = compileShader "VS" vs
                              return Some vsc
  
negative.code:     let private compileEffectInternal (e : Compiled<Effect, ShaderState>) =
          compile {
              let! e = e
              let! glslVersion = version
  
              let hasgs = match e.geometryShader with | Some _ -> true | _ -> false
              let topUsed = ["Colors", typeof<V4d>; "Depth", typeof<float>] |> Map.ofList
  