<xsl:template match="//topic">

starting a match with // never does anything useful (in XSLT1 it just
changes the default priority, not the elements that are matched, in
XSLT2 it does that and also slows down processing as it causes the
processor to check that the element is contained in a document).

Presumably your real example doesn't have topic at the top? in which
case you just need match="topic"

David