-- | Two records may be pasted together.
rappend
  :: Rec f as
  -> Rec f bs
  -> Rec f (as ++ bs)
rappend RNil ys = ys
rappend
  :: Rec f as
  -> Rec f bs
  -> Rec f (as ++ bs)
rappend RNil ys = ys
rappend RNil ys = ys