readonly proc _listf (nm, elt, vis) is
    local list = nm & "List"
    local listf = nm & "ListF"
    build_generic_intf (list, "List", [elt], vis)
    build_generic_impl (list, "List", [elt])
    build_generic_intf (listf, "ListF", [elt, list], vis)
    build_generic_impl (listf, "ListF", [elt, list])
  end

readonly proc Listf (nm, elt) is _listf (nm, elt, VISIBLE) end
readonly proc listf (nm, elt) is _listf (nm, elt, HIDDEN)  end