<Urmel>
  <T>0</T>
  <T>6</T>
  <T>12</T>
  <T>18</T>
  <T>24</T>
  <B>15</B>
  <B>6</B>
  <B>7</B>
  <B>12</B>
  <B>9</B>
  <B>10</B>
</Urmel>

A <T> indicates a point in time. <T>s are ordered numerically. Direct
siblings define an interval starting at the earlier <T> and ending
before the later <T>.

I want to find all <T>s that start an interval which includes at least
one event. All <T>s except the last one start an interval. The next
following sibling ends the interval. This is easy. If the count of <B>s
for the interval is zero, the interval is empty and I'm not interested;
else I want it in the output. Determining emptiness isn't easy - yet.

The following produces empty output and hence doesn't work, but you
probably can see my intention. Using the current() function, I'm trying
to refer to the current <T> to find out if the current <B> contained in
$b falls within the bounds of the interval, but without success.

This may fail because I'm nesting predicates and would need another
variable instead of the current() function, but I can't figure out how
to solve this, and maybe the problem can be reframed more intelligently.

<xsl:transform version="1.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="text"/>
  <xsl:template match="/">
    <xsl:variable name="b" select="//B"/>
    <xsl:apply-templates select="
      //T [ position() != last() ]
          [ count( $b[.  >=  current() and
                      . &lt; current()/following-sibling::T[1]]) > 0]"/>
  </xsl:template>
  <xsl:template match="T">
    <xsl:value-of select="."/>
    <xsl:text>&#10;</xsl:text>
  </xsl:template>
</xsl:transform>