比如说:
文件1:
<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE categories [
  <!ELEMENT categories (category*) >
  <!ELEMENT category (title) >
  <!ELEMENT title (#PCDATA) >
<!ATTLIST category category_id ID #REQUIRED >  这里定义了category id
]>
<categories>
  <category category_id='c1'>
    <title>XML 1</title>
  </category>
  <category category_id='c2'>
    <title >XML 2</title>
  </category>
  <category category_id='c3'>    c3在这里
    <title>XML 3</title>
  </category>
</categories>

文件2:
<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE article [
<!ELEMENT articles (article, feedback*)>
<!ELEMENT article (title)>
<!ATTLIST article article_id ID #REQUIRED>
[color=#FF0000]<!ATTLIST article category_id IDREF #REQUIRED> 这里引用了category id[/color]
<articles>
  <article article_id='a1' category_id='c3'>   引用 c3
    <title>xml reader usage </title>
</article>
</articles>

因为引用category id 和定义category id 不在同一个文件中, 所以验证文件2时会报"找不到c3"的错误.

请问有什么办法引用吗?