basic xsl question

Stylesheet like this:

  <xsl:for-each select="root/orders/order">
    <!-- the person is '/root/order/person_id' in root/persons/person/id
-->
    <xsl:value-of select="root/persons/person[id=person_id]/name" />
  </xsl:for-each>


The problem i'am having is related to the value-of row. How do i write that
so it works? 

root/persons/person[id=person_id]/name

id -         is persion id
person_id -  is the person of an order specifyed by an id

If i write it like that the processor will try to compare person/id with
person/person_id but there is no person_id under person, it's under order.

help pls
  Matts Isuls

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list