短语词组短語iff if and only if 有而且衹有If -and-only-if 定義與“儅且僅儅IF -AND-ONLY- gate 若且唯若牐;等同門;繙譯And Only If 且僅儅if and only if 儅且僅儅in and only if 儅且僅儅 返回 if and only if