shrinkFE
    :: PreOpenFun acc env aenv f
    -> PreOpenFun acc env aenv f
shrinkFE fun =
  case fun of
    Body e      -> Body (shrinkE e)
    Lam f       -> Lam (shrinkFE f)
shrinkFE
    :: PreOpenFun acc env aenv f
    -> PreOpenFun acc env aenv f
shrinkFE fun =
  case fun of
    Body e      -> Body (shrinkE e)
    Lam f       -> Lam (shrinkFE f)
shrinkFE fun =
  case fun of
    Body e      -> Body (shrinkE e)
    Lam f       -> Lam (shrinkFE f)