prettyBoundary :: forall acc aenv dim e. Elt e
               => {-dummy-}acc aenv (Array dim e) -> Boundary (EltRepr e) -> Doc
prettyBoundary _ Clamp        = text "Clamp"
prettyBoundary :: forall acc aenv dim e. Elt e
               => {-dummy-}acc aenv (Array dim e) -> Boundary (EltRepr e) -> Doc
prettyBoundary _ Clamp        = text "Clamp"
prettyBoundary _ Clamp        = text "Clamp"