Selecting the top-most ancestor.

I am wanting the select the top-most ancestor of an element and then be able to process back down the document tree. The basic structure of the document to be processed is as follows:

<system-index-block>
<system-folder>
<system-page/>
<system-page/>
</system-folder>
<system-folder>
<system-folder>
<system-page current="true"/>
</system-folder>
<system-page/>
</system-folder>
<system-page/>
<system-page/>
</system-index-block>

I can get to the system-page with attribute current set to true. But what do I need to do to get back to the top-most system-folder element that is within root. I used a <xsl:for-each select="ancestor::system-folder"> to produce a bread crumb trail but didn't know how I might be able to limit such a select more to just the ancestor::system-folder whose parent is root/system-index-block...

Any Hints?