<xsl:apply-templates select="
     //T [ position() != last() ]
         [ count( $b[.  >=  current() and
                     . &lt; current()/following-sibling::T[1]]) > 0]"/>

You don't need to use count() > 0 to check for existence... just do:

//T [ position() != last() ]
  [$b[.  >=  current() and . < current()/following-sibling::T[1]]]

That's true. All I want is a boolean, and I don't need the count
for that, existence is sufficient. Thanks!