1<?xml version="1.0"?>
2<!DOCTYPE main [
3 <!ELEMENT main (a|b)*>
4 <!ELEMENT a (#PCDATA)>
5 <!ATTLIST a id ID #REQUIRED>
6 <!ELEMENT b (#PCDATA)>
7]>
8<main>
9 <a id="w">W</a>
10 <a id="x">X</a>
11 <a id="y">Y</a>
12 <a id="z">Z</a>
13 <b>y</b>
14 <b>w</b>
15 <b>x</b>
16</main>