Insert an new Element on a attribute value change / test for nonexistenc

Hi there,

I'm stuck with a transformation problem that I can't seem to solve. I need 
to insert a hierarchy level based on an attribute value I have 
(simplified):
<?xml version="1.0" encoding="UTF-8"?>
<Book>
        <Pages>
                <page picture="pic1.jpg" newchapter="true" Title="One upon 
the time" StartPage="5">Once upon the time there was a king</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic1.jpg" newchapter="true" Title="The Test
" StartPage="11">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic1.jpg" newchapter="true" Title="The 
Quest" StartPage="36">Bla Bla Bla</page>
                <page picture="pic1.jpg" newchapter="true" Title="The 
Happy end" StartPage="37">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
                <page picture="pic2.jpg">Bla Bla Bla</page>
        </Pages>
</Book>

I want:
<?xml version="1.0" encoding="UTF-8"?>
<Book>
        <Chapters>
                <Chapter Title="One upon the time" StartPage="5">
                        <page picture="pic1.jpg">Once upon the time there 
was a king</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                </Chapter>
                <Chapter Title="The Test" StartPage="11">
                        <page picture="pic1.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                </Chapter>
                <Chapter Title="The Quest" StartPage="36">
                        <page picture="pic1.jpg">Bla Bla Bla</page>
                </Chapter>
                <Chapter Title="The Happy end" StartPage="37">
                        <page picture="pic1.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                        <page picture="pic2.jpg">Bla Bla Bla</page>
                </Chapter>
        </Chapters>
</Book>


I tried:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="
http://www.w3.org/1999/XSL/Transform";>
        <xsl:output method="xml" version="1.0" encoding="UTF-8" indent="
yes"/>
        <xsl:template match="/">
                <xsl:apply-templates/>
        </xsl:template>
        <xsl:template match="Book">
                <Book>
                        <xsl:apply-templates/>
                </Book>
        </xsl:template>
        <xsl:template match="Pages">
                <Chapters>
                        <xsl:apply-templates select="
page[(_at_)newchapter='true']"/>
                </Chapters>
        </xsl:template>
        <xsl:template match="page[(_at_)newchapter='true']">
                <xsl:element name="Chapter">
                        <xsl:attribute name="Title"><xsl:value-of select="
@Title"/></xsl:attribute>
                        <xsl:attribute name="StartPage"><xsl:value-of 
select="@StartPage"/></xsl:attribute>
                        <xsl:element name="page">
                                <xsl:attribute name="picture"><
xsl:value-of select="@picture"/></xsl:attribute>
                                <xsl:value-of select="."/>
                        </xsl:element>
                        <!-- Here I got stuck: how to get all following 
siblings until there is one with a startpage how to test for non existance 
of an attribute? and how to get until the next similar-->
                        <xsl:apply-templates select="
following-sibling::page[(_at_)newchapter='']"/>
                </xsl:element>
        </xsl:template>
        <xsl:template match="page">
                <xsl:element name="page">
                        <xsl:attribute name="picture"><xsl:value-of select
="@picture"/></xsl:attribute>
                        <xsl:value-of select="."/>
                </xsl:element>
        </xsl:template>
        <xsl:template match="*"/>
</xsl:stylesheet>


There are 2 problems that I face: How to test for the non-existance of an 
attribute. And how to select a subset of the following siblings. 
The rule: all following siblings until you find one with an existing 
attribute newchapter='true'.

Any hint what I can do?
Thx
;-) stw