abstract grammar model(AGM): common part.

## ID/IDREF support

Support for 'ID' and 'IDREF' types are implemented in this package, due to the restriction of the implementation.

The implementation in this package works as follows:

- When verify method of IDType is called, it only checks the conformance as a NMTOKEN and uniqueness, then stores it as ID.
- When verify method of IDREFType is called, it only checks the conformance as a NMTOKEN and stores it as IDREF.
- When the entire document is parsed, ID list and IDREF list are cross-checked. IDREF without the corresponding ID is detected in this stage.

This implementation implies very severe limitations. For example, consider the following TREX pattern.

    <element name="foo">
      <choice>
        <attribute name="bar" type="ID" />
        <attribute name="bar" type="string" />
      </choice>
      <element name="child">
        <attribute name="id" type="ID" />
      </element>
    </element>

And the following XML fragment.

    <foo bar="xyz"> <child id="xyz" /> </foo>

This implementation cannot validate the above fragment, because "xyz" is registered as an ID when "bar" attribute is tested against "ID" type. IDREF type also has similar problems.

Note that RELAX prohibits these "abuse" of ID/IDREF in the way that this implementation works.