Explicit requirements for rdf:langString and rdf:dirLangString - alternative

[pchampin]

alternative to #212

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[afs]

"optional" ... "MUST be present if and only if" does not read very well. It's not an option.

[TallTed]

tweaks for clarity

[afs]

Suggested change

	<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>,
	It MUST be well-formed according to
	<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>
	and it MUST be well-formed according to

[pchampin]

Suggested change

	<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>,
	It MUST be well-formed according to
	<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>.
	It MUST be well-formed according to

I would rather cut the sentence, because the resulting sentence would be very long, and it alteady contains an "and" (line 941).

[TallTed]

As @afs said, base direction is not really optional, because of the conditional requirement ("MUST be present if and only if"). Noodling on rephrasing --

      <li>A conditionally required <dfn>base direction</dfn>.
        It MUST be present
        IF AND ONLY IF the <a>datatype IRI</a> is
        <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>,
        when it MUST be one of the following:<ul>
          <li>`ltr`, indicating that the initial text direction is set to left-to-right</li>
          <li>`rtl`, indicating that the initial text direction is set to right-to-left</li>
        </ul></li>

It might be worth another line saying something like, It MUST NOT be present if the <a>datatype IRI</a> is anything other than <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>. This could be a NOTE or otherwise "quieter", as it's just reinforcing what has already been said by the (intentionally emphasized) IF AND ONLY IF.