luaL_checknumber
return luaL_opt(L, luaL_checknumber, narg, def);
LUALIB_API lua_Number (luaL_checknumber) (lua_State *L, int numArg);
lua_Number n = luaL_checknumber(L, arg);
lua_Number n = luaL_checknumber(L, arg);
nb = str_sprintf(buff, form, (LUA_FLTFRM_T)luaL_checknumber(L, arg));