[tei-council] <date>, <distance>, and <measure>

[Syd Bauman]

> If you believe the attList specification allows what you need, then
> I should implement it. I'm willing to have a try, if its needed.

I certainly think the <attList> specification allows this:

  <elementSpec ident="duck">
    <attList org="choice">
      <attList org="group">
        <attDef ident="A1" usage="req"/>
        <attDef ident="A2" usage="req"/>
      </attList>
      <attList org="group">
        <attDef ident="B1" usage="req"/>
        <attDef ident="B2" usage="req"/>
      </attList>
    </attList>
  </elementSpec>

This states that a <duck> must have either both A1= and A2= or both
B1= and B2=, and can't have all four of them (or zero, one, or three
of them).

This could be expressed in RelaxNG as

    <rng:element name="duck">
      <rng:choice>
        <rng:group>
          <rng:attribute name="A1">
            <rng:empty/>
          </rng:attribute>
          <rng:attribute name="A2">
            <rng:empty/>
          </rng:attribute>
        </rng:group>
        <rng:group>
          <rng:attribute name="B1">
            <rng:empty/>
          </rng:attribute>
          <rng:attribute name="B2">
            <rng:empty/>
          </rng:attribute>
        </rng:group>
      </rng:choice>
      <rng:empty/>
    </rng:element>

which in the compact syntax requires what always looks to me like an
extra layer of parenthesis, but makes sense when you think of the
outer parens as <rng:choice> and the inner parens as <rng:group>:

  element duck {
    (
      ( attribute A1 { empty }, attribute A2 { empty } )
      |
      ( attribute B1 { empty }, attribute B2 { empty } )
    ),
    empty
  }

What I'm not certain of is whether we want to change the
implementation to do what the specification permits, or change the
specification so that you can't do this. I lean towards the former,
but it may be more effort than it's worth.


> On the other hand, restrictions like this are good candidates for
> Schematron, as you say.

True. And although there are some disadvantages with using Schematron
(speed problems jump to mind), it does have the advantage of
providing a check for those who use DTDs.