luaL_optlstring
#define luaL_optstring(L,n,d) (luaL_optlstring(L, (n), (d), NULL))
LUALIB_API const char *(luaL_optlstring) (lua_State *L, int numArg,
const char *sep = luaL_optlstring(L, 3, "", &lsep);
const char *sep = luaL_optlstring(L, 2, "", &lsep);