prettyPreExp :: forall acc t env aenv.
                PrettyAcc acc -> Int -> Int -> (Doc -> Doc) -> PreOpenExp acc env aenv t -> Doc
prettyPreExp _pp lvl _ _ (Var idx)
  = text $ 'x' : show (lvl - idxToInt idx)
prettyPreExp :: forall acc t env aenv.
                PrettyAcc acc -> Int -> Int -> (Doc -> Doc) -> PreOpenExp acc env aenv t -> Doc
prettyPreExp _pp lvl _ _ (Var idx)
  = text $ 'x' : show (lvl - idxToInt idx)
prettyPreExp _pp lvl _ _ (Var idx)
  = text $ 'x' : show (lvl - idxToInt idx)