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[11] [DFN[[['''MAY''']]]]、[DFN[[['''OPTIONAL''']]]]、[DFN[[['''OPTIONALLY''']]]] [SRC[>>12]]
([DFN[[['''構わない''']]]]、[DFN[[['''任意選択''']]]])
は、その対象が任意選択であり、従っても従わなくても[[適合]]することを表す[[助動詞]]です。

* 仕様書

[REFS[
- [9] '''[CITE@en[RFC 2119 - Base Encodings]] ([TIME[2014-06-11 10:38:58 +09:00]] 版) <http://tools.ietf.org/html/rfc2119#section-5>'''
- [12] [CITE@en-GB-x-hixie[HTML Standard]] ([TIME[2014-06-20 15:40:17 +09:00]] 版) <http://www.whatwg.org/specs/web-apps/current-work/#conformance-requirements>
]REFS]

* 定義

[10] [['''MAY''']]、[['''OPTIONAL''']] は、その項目が真に任意選択であることを表します。
ある実装者は市場の要求に基づき、あるいはその項目を実装しない他の実装者の製品よりも良い製品を提供するために、
これを含めることを選ぶかもしれません。その項目を選択しない実装は、
選択する実装と、機能は不足するかもしれませんが、[[相互運用]]可能でなければ[['''なりません''']]。
その項目を選択する実装は、選択しない実装と、もちろん当該任意選択機能を除いて[[相互運用]]可能であるようにしなければ[['''なりません''']]。
[SRC[>>9]]

* 他の定義

[14] 
>
:[RUBYB[して構わない][may]]:
[Q[して構わない]]という語は、[[実装]]に関しては、
この仕様においては[[任意選択]]の機能であり、
必須ではないものの提供しても構わないと解釈します。
[[文書]]の適合性に関しては、
その[[任意選択]]の機能を使用してはならないことを意味します。
[Q[[RUBYB[[[任意選択]]][[[optional]]]]]]という語は[Q[して構わない]]と同じ定義とします。

[REFS[
- [15] [[XHTML 1.0]]
--[CITE[2.1. Terminology]] <IW:XHTML10:"terms">
]REFS]

[13] [CITE@EN[XQuery 1.0 and XPath 2.0 Functions and Operators (Second Edition)]]
( ([TIME[2010-12-17 00:06:54 +09:00]] 版))
<http://www.w3.org/TR/2010/REC-xpath-functions-20101214/#may>

* 歴史

[FIG[
[3] 
>
:"MAY":
This word or the adjective "OPTIONAL" means that this item
is truly optional.  One vendor may choose to include the
item because a particular marketplace requires it or
because it enhances the product, for example; another
vendor may omit the same item.

[FIGCAPTION[
[CITE@en[RFC 1123 - Requirements for Internet Hosts - Application and Support]] ([TIME[2010-07-22 04:07:59 +09:00]] 版) <http://tools.ietf.org/html/rfc1123#page-11>
]FIGCAPTION]
]FIG]

[FIG[
[5] 
>The key word "OPTIONALLY" in
the normative parts of this document is to be interpreted with the
same normative meaning as "MAY" and "OPTIONAL".

[FIGCAPTION[
[CITE@en[Web Applications 1.0 r6966     Allow browsers to bail early for showModalDialog, alert, confirm, and prompt during pagehide, beforeunload, and unload events.]] ([TIME[2012-02-07 07:54:00 +09:00]] 版) <http://html5.org/tools/web-apps-tracker?from=6965&to=6966>
]FIGCAPTION]
]FIG]

* メモ

[1]
[CITE@en[XML Query (XQuery) Requirements]] ([CODE[2007-03-17 06:39:09 +09:00]] 版) <http://www.w3.org/TR/2007/NOTE-xquery-requirements-20070323/#terminology-may>
([[名無しさん]])

[2]
[CITE@en[XML Query (XQuery) 1.1 Requirements]] ([TIME[2007-03-16 09:42:40 +09:00]] 版) <http://www.w3.org/TR/2007/WD-xquery-11-requirements-20070323/#terminology-may>
([[名無しさん]])

[4] [CITE@en[Web Applications 1.0 r6966     Allow browsers to bail early for showModalDialog, alert, confirm, and prompt during pagehide, beforeunload, and unload events.]]
( ([TIME[2012-02-07 07:54:00 +09:00]] 版))
<http://html5.org/tools/web-apps-tracker?from=6965&to=6966>

[6] [CITE@EN[W3C XML Schema Definition Language (XSD) 1.1 Part 2: Datatypes]]
( ([TIME[2012-04-05 06:34:51 +09:00]] 版))
<http://www.w3.org/TR/2012/REC-xmlschema11-2-20120405/#dt-may>

[7] [CITE@EN[W3C XML Schema Definition Language (XSD) 1.1 Part 1: Structures]]
( ([TIME[2012-04-05 06:34:07 +09:00]] 版))
<http://www.w3.org/TR/2012/REC-xmlschema11-1-20120405/#intro-terminology>

[8] [CITE@EN[XPath and XQuery Functions and Operators 3.0]]
( ([TIME[2014-04-08 07:02:07 +09:00]] 版))
<http://www.w3.org/TR/xpath-functions-3/#may>

[FIG(quote)[
[FIGCAPTION[
[16] [CITE@en[RFC 2911 - Internet Printing Protocol/1.1: Model and Semantics]]
([TIME[2015-02-15 17:22:27 +09:00]] 版)
<https://tools.ietf.org/html/rfc2911#section-12.1.1>
]FIGCAPTION]

> The verb "NEED NOT" indicates an action that the subject of the sentence does not have to implement in order to claim conformance to the standard.  The verb "NEED NOT" is used instead of "MAY NOT" since "MAY NOT" sounds like a prohibition.

]FIG]

[FIG(quote)[
[FIGCAPTION[
[17] [CITE@en[Requirements for the Evaluation and Report Language (EARL) 1.0]]
([TIME[2017-02-01 07:17:48 +09:00]])
<https://www.w3.org/TR/2017/NOTE-EARL10-Requirements-20170202/>
]FIGCAPTION]

> MAY
> The collection of specifications and technical notes that comprise the EARL 1.0 vocabulary produced by the Working Group can exclude this requirement.

]FIG]

[18] [CITE@EN[XQuery 3.1: An XML Query Language]]
([TIME[2017-03-20 09:23:33 +09:00]])
<https://www.w3.org/TR/2017/REC-xquery-31-20170321/#id-conformance>

[19] [CITE@EN[XML Path Language (XPath) 3.1]]
([TIME[2017-03-20 09:18:04 +09:00]])
<https://www.w3.org/TR/2017/REC-xpath-31-20170321/#may>

[20] [CITE@EN[XPath and XQuery Functions and Operators 3.1]]
([TIME[2017-03-21 16:02:06 +09:00]])
<https://www.w3.org/TR/2017/REC-xpath-functions-31-20170321/#conformance-terminology>

[FIG(quote)[
[FIGCAPTION[
[21] [CITE@en-US[ARIA in HTML]]
([TIME[2017-09-10 07:44:56 +09:00]])
<https://w3c.github.io/html-aria/#conformance-requirements>
]FIGCAPTION]

>  The key word "OPTIONALLY" in the normative parts of this document is to be interpreted with the same normative meaning as "may" and "optional".

]FIG]

[22] [CITE@en[Infra Standard]]
([TIME[2018-09-13 23:21:15 +09:00]])
<https://infra.spec.whatwg.org/#conformance>

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