[xsl] Option mystery

["Oleg Konovalov" <olegkon@xxxxxxxxx>]

Hi,
I am trying to fix a bug in somebody else's XSLT1.0/Cocoon2.0.4 app.
I encountered something in the code which I don't understand.

<xsl:choose>
 <xsl:when test="$tier/option[option-num=$cur-option-num and
@rate-increase='true']">
   <option rate-increase="{@rate-increase}">
      <debug1>branch1</debug1>
       ...other elements
    </option>
  </xsl:when>
</xsl:choose>

A few questions regarding snippet:
1) is that "option" a predefined XSLT1.0 construct / keyword / operation ?
I can't find any in the XSLT book.
Could you please explain it to me what that is (in details).

2) the bug I am trying to fix is that somehow on exit I am getting a node
with <option rate-increase="false"> (that debug1=branch1 and
option-num=1).  How is that possible ?   That "branch1" is unique and
can't come from any other place. How can that expression inside []
fail ?


Sorry for the newbie questions.

Thank you in advance,
Oleg.