<book>
<author>author1</author>
<name>x box</name>
<type>sc-fi</type>
</book>
<book>
<author>author2</author>
<name>elf story</name>
<type>adventure</type>
</book>
<book>
<author>author1</author>
<name>the return of Dr who</name>
<type>sc-fi</type>
</book>
<book>
<author>author2</author>
<name>something2</name>
<type>sc-fi</type>
</book>
...
say i was looking for all the type of books written by aurthor2, using the
following call the processor will skip the type which it sees before (e.g.
sc-fi will not be shown as it already appears previously for author1)