l_jail_iter
struct l_jail_iter *iter, **iterp;
iterp = (struct l_jail_iter **)lua_touserdata(L, 1);
struct l_jail_iter *iter;
*(struct l_jail_iter **)lua_newuserdata(L,
sizeof(struct l_jail_iter **)) = iter;
struct l_jail_iter *iter, **iterp;
iterp = (struct l_jail_iter **)luaL_checkudata(L, 1, JAIL_METATABLE);