XSL - Check nodes at the same level

I have this structure in XML:

<report>
<text>
    <chapter>1</chapter>
    <chapter>2
        <section>2.1</section>
    </chapter>
    <chapter>3</chapter>
</text></report>

In my DTD, I have:

 <!ELEMENT text (#PCDATA | chapter |section)*>

Because of this, in my XSL, i want the garantee that I don't have a chapter and a section in the same level, before doing the transformation. So, i can't have this

<report>
<text>
    <chapter>1</chapter>
    <chapter>2
        <section>2.1</section>
    </chapter>
    <section>3</section>
</text></report>

My XSL

<xsl:template match="chapter">

    <div class="chapter">   
        <h2> 
            <xsl:apply-templates select="chapter-title"/>
        </h2>       
        <hr/>
        <xsl:apply-templates select="text() | section "/>
    </div>
</xsl:template> 
<xsl:template match="section">
    <div class="section">
        <h3>  
            <xsl:apply-templates select="section-title"/>
        </h3>
        <xsl:apply-templates select="text()"/>
    </div>
</xsl:template>

How can I check if a section and a chapter are at the same level, in XSL? Thanks.