rank/rank0
<!DOCTYPE test [
<!ELEMENT test - - ((h1|h2|h3|p1|p2|p3)+)>
<!ELEMENT (h|p) 1 - - (#PCDATA)>
<!ELEMENT (h|p) 2 - - (#PCDATA)>
<!ELEMENT (h|p) 3 - - (#PCDATA)>
]>
<test>
<h1>Heading 1</h1>
<p>Paragraph 1</p>
<h2>Heading 1.1</h2>
<p>Paragraph 1.1</p>
<h3>Heading 1.1.1</h3>
<p>Paragraph 1.1.1</p>
</test>