ancestors property?

If I have something like this:

<outside myprop='123'>
  <mid>
    <inside>456</inside>
  </mid>
</outside>


How would I write my match for my <xsl:template> for the <inside> node so
that that template would only match if outside's myprop was equal to '123'
(and another template might match only '234' for example. I want there to
be any number of nested levels between the <outside> and <inside> tags. I
tried using ancestor() but I must have messed something up because it said
ancestor didn't exist.

Thanks.

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