Transformation example - hierarchical document

<!DOCTYPE DOC [
<!ELEMENT DOC - O (P+)> 
<!ELEMENT P - O (#PCDATA|L)+    -- Paragraphs -->
<!ELEMENT L - O (I+)            -- Lists      -->
<!ELEMENT I - O (P+)            -- List items -->
]>
<DOC>
<P>Introductory paragraph</P>
<P>Second paragraph :
<L>
<I><P>First case</P><P>Second paragraph in item</P></I>
<I><P>Second case :
<L>
<I><P>First sub-case</P></I>
<I><P>Second sub-case</P></I>
</P>
</I>
Continuation of second paragraph
</P>
</DOC>