<!DOCTYPE doc
[
<!ELEMENT doc (a|refs)*>
<!ELEMENT a EMPTY>
<!ELEMENT refs EMPTY>
<!ATTLIST refs refs IDREFS #REQUIRED>
<!ATTLIST a id ID #REQUIRED>
]>
<doc>
<a id="A1"/><a id="A2"/><a id="A3"/>
<refs refs="A1 A2 A3"/>
<refs refs="A1
A2	A3"/>
<refs refs="A1"/>
</doc>