selecting following attribute

I am creating a specialized table of contents that will contain page ranges
(pp. 5-12, p. 17). I am starting with a flat structure in which the page
reference for each entry has been changed to a @startNum attribute:

<Book>
<UnitName startNum="2">Unit Name</UnitName>
<chapter startNum="4" number="1">
   <title>Chapter 1: ...</title>
</chapter>
<lesson startNum="5">...</lesson>
<sublesson startNum="5">1.	 ...</sublesson>
<sublesson startNum="7">2.	 ...</sublesson>
<sublesson startNum="8">3.	...</sublesson>
<sublesson startNum="12">4.	 ...</sublesson>
<lesson startNum="13">...</lesson>
<sublesson startNum="13">1.	 ...</sublesson>
<italicFeature singlePage="17">...</italicFeature>
<sublesson startNum="18">2.	 ...</sublesson>
<sublesson startNum="21">3.	 ...</sublesson>
<sublesson startNum="22">4.	 ...</sublesson>
<lesson startNum="24">...</lesson>
<italicFeature singlePage="26">...</italicFeature>
<sublesson startNum="29">5.	 ...</sublesson>
<lesson startNum="33">...</lesson>
<chapter startNum="34" number="2">
   <title>Chapter 2: ...</title>
</chapter>
<!--much more like this-->
</Book>

The desired output would add an endNum attribute to each element:

<chapter startNum="4" number="1" endNum="33">
   <title>Chapter 1: ...</title>
</chapter>
<lesson startNum="5" endNum="12">...</lesson>

Issues:

Each <italicFeature> element is single page entry that must be included in the
sublesson page range.

 <sublesson startNum="13" endNum="17">1.	 ...</sublesson>
<italicFeature singlePage="17">...</italicFeature>
<sublesson startNum="18">2.	 ...</sublesson>

My current strategy for generating the endNum attributes is to subtract 1 from
the following startNum if it satisfies certain conditions. In this template, I
first test to see if the following @startNum is equal to the current
@startNum. If it is, then the @endNum will be the same as the @startNum. If it
isn't, then I need to subtract 1 from the following @startNum.

   <xsl:template match="sublesson">
        <xsl:choose>
            <xsl:when test="following::*/@startNum[1] = @substartNum">
                <xsl:copy>
                    <xsl:copy-of select="@*"/>
                    <xsl:attribute name="endNum">
                        <xsl:value-of select="@startNum"/>
                    </xsl:attribute>
                    <xsl:apply-templates/>
                </xsl:copy>
            </xsl:when>
            <xsl:otherwise>
                <xsl:copy>
                    <xsl:copy-of select="@*"/>
                    <xsl:attribute name="endNum">
                        <xsl:value-of select="following::*[1]/@startNum -1"/>
<!--offending line-->
                    </xsl:attribute>
                    <xsl:apply-templates/>
                </xsl:copy>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>

All well and good except that the offending line noted above does not seem to
select the first following @startNum. Here is a snippet of my output:

<lesson startNum="13" endNum="23"/>...</lesson>
      <sublesson startNum="13" endNum="">1.	 ...</sublesson>     <!--no
endNum-->
      <italicFeature singlePage="17">C...</italicFeature>
      <sublesson startNum="18" endNum="20">2.	...</sublesson>

My question: How do I select the first following @startNum?

This formulation

<xsl:value-of select="following::*/@startNum[1] -1"/>

returns this error in Oxygen: Saxon B 9.0.0.6: A sequence of more than one
item is not allowed as the first operand of '-'

Of course, my approach to this problem could be the problem. Perhaps there is
a better approach. I have tried grouping the chapters and lessons first, but
the problem noted above still remains.

Terry