XSLT: Looping and setting a variable when condition met at least once - variables

Question

I am trying to loop through a node, which in itselt is not a problem. I want to check to see if the selectedLetter matches the first letter of the node, which is working as intended (see code below). The difficulties arise in that I would like to present some alternative text if the conditional "" is not met at least once during the loop, along the lines of "Sorry, no documents begin with that letter". I was thinking about setting a variable i.e. flag in the conditional and check later on if this variable has been set, but from what I have read here, the scope of the variable is restricted and only available within the loop? Can anyone offer any assistance?
Cheers,
Taff
<xsl:param name="selectedLetter" select="''"/>
<xsl:key name="docLetter" match="docs" use="substring(text()),1,1)"/>
<xsl:for-each select="//ad-documents/item">
<xsl:sort select="documentName"/>
<xsl:variable name="firstLetter" select="upper-case(substring(documentName, 1, 1))"/>
<xsl:choose>
<xsl:when test="$firstLetter = $selectedLetter">
<div class="doc-date">
<xsl:if test="upload-date/day < 10">0</xsl:if>
<xsl:value-of select="upload-date/day"/>
<xsl:text>.</xsl:text>
<xsl:if test="upload-date/month < 10">0</xsl:if>
<xsl:value-of select="upload-date/month"/>
<xsl:text>.</xsl:text>
<xsl:value-of select="upload-date/year"/>
</div>
<div class="doc-link">
<a>
<xsl:attribute name="href">
<xsl:choose>
<xsl:when test="isEditableFormDoc/box = 'true'">
#
</xsl:when>
<xsl:otherwise>
<xsl:text>/img/ejbfile/</xsl:text>
<xsl:value-of select="documents/document/#documentName"/>
<xsl:text>?id=</xsl:text>
<xsl:value-of select="documents/document/#src"/>
</xsl:otherwise>
</xsl:choose>
</xsl:attribute>
<xsl:if test="isEditableFormDoc/box = 'true'">
<xsl:attribute name="target">
<xsl:text>_blank</xsl:text>
</xsl:attribute>
</xsl:if>
<xsl:value-of select="documentName"/>
</a>
</div>
<div class="doc-short-desc">
<xsl:apply-templates select="document-short-desc" mode="format"/>
</div>
<!--<xsl:apply-templates select="//ad-documents/item" mode="filteredDocuments"/>-->
</xsl:when>
</xsl:choose>
</xsl:for-each>
EDIT:
XML Example
<ad-documents name="ad-documents" label="Dokumente (limitiert auf 20 pro Seite)">
<item description="Doc%203" id="1" timestamp="1328525592205">
<documentName name="documentName" description="Doc%203">Doc 3</documentName>
<optionalLetter name="optionalLetter" description="c">c</optionalLetter>
<document-short-desc name="document-short-desc" description="">
<p>yxf</p>
</document-short-desc>
<upload-date name="upload-date" description="">
<day>24</day>
<month>2</month>
<year>2012</year>
</upload-date>
<isEditableFormDoc name="isEditableFormDoc" description="">
<box/>
</isEditableFormDoc>
<documents name="documents" description=""/>
</item>
<item description="Doc%204" id="2" timestamp="1328525624889">
<documentName name="documentName" description="Doc%204">Doc 4</documentName>
<optionalLetter name="optionalLetter" description="z%2Ci%2Cg">z,i,g</optionalLetter>
<document-short-desc name="document-short-desc" description="">
<p>asff</p>
</document-short-desc>
<upload-date name="upload-date" description="">
<day>25</day>
<month>2</month>
<year>2012</year>
</upload-date>
<isEditableFormDoc name="isEditableFormDoc" description="">
<box/>
</isEditableFormDoc>
<documents name="documents" description=""/>
</item>
<item description="Doc%201" id="1" timestamp="1328523551639">
<documentName name="documentName" description="Doc%201">Doc 1</documentName>
<optionalLetter name="optionalLetter" description="b%2Cc">b,c</optionalLetter>
<document-short-desc name="document-short-desc" description="">
<p>Short Desc 1</p>
</document-short-desc>
<upload-date name="upload-date" description="">
<day>9</day>
<month>2</month>
<year>2012</year>
</upload-date>
<isEditableFormDoc name="isEditableFormDoc" description="">
<box/>
</isEditableFormDoc>
<documents name="documents" description=""/>
</item>
</ad-documents>

Answer 1

The solution is simple:
Just replace:
<xsl:for-each select="//ad-documents/item">
with
<xsl:for-each select=
"//ad-documents/item
[$selectedLetter eq upper-case(substring(documentName, 1, 1) ]">
You can add after this xsl:for-each
<xsl:sequence select=
"'YourErrorMessage'
[not(//ad-documents/item
[$selectedLetter eq upper-case(substring(documentName, 1, 1) ]
)
]"
/>

Answer 2

Thanks to Dimitre, I managed to solve the problem with count()
<xsl:variable name="foundDocs" select="count(//ad-documents/item[$selectedLetter eq upper-case(substring(documentName, 1, 1))])"/>
<xsl:if test="$foundDocs = 0">
<div class="no-documents">
<xsl:text>Leider keine Dokumente gefunden</xsl:text>
</div>
</xsl:if>