Why my Xquery always return an empty set?

I tried to execute Xquery following the style that in basic Xquery samples, 
however, no matter I tried to execute the Xquery for my XML file, or Xquery 
samples in basic Xquery section from web page, both of them return an empty 
set, What is problem for my Xquery? Previously I execute the Xquery for another 
XML file, and it looked OK.

Thanks.

Sam


for $StillRegion in /Mpeg7//StillRegion[Text&='tree']
return
        <result>
                {$StillRegion}
        </result>


XML file

<Mpeg7 xmlns="urn:mpeg:mpeg7:schema:2001" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"; 
xmlns:mpeg7="urn:mpeg:mpeg7:schema:2001" 
xsi:schemaLocation="urn:mpeg:mpeg7:schema:2001 Mpeg7-2001.xsd">

<!-- Your MPEG-7 Metadata -->

<Description xsi:type="ContentEntityType">
        <MultimediaContent xsi:type="MultimediaCollectionType">
                <Collection xsi:type="ContentCollectionType">
                        <Content xsi:type="ImageType">
                                <Image>
                                        <MediaLocator 
xsi:type="ImageLocatorType">
                                        
        <MediaUri>MyImage.jpg</MediaUri> 
                                        </MediaLocator>
                                        <SpatialDecomposition gap="true" 
overlap="false">
                                                <StillRegion id="1">
                                                        <DescriptionUnit 
xsi:type="HistogramProbabilityType" 
                                                                                
        momentNormalized="1" dim="8">
                                                                <Distribution 
mpeg7:dim="8">
                                                                        0.125 
0.0 0.0 0.25 0.125 0.25 0.25 0.0
                                                                </Distribution> 
                                                        </DescriptionUnit>
                                                        <SpatialLocator>
                                                                <Polygon>
                                                                        <Coords 
mpeg7:dim="2 5"> 5 25 10 20 15 15 10 10 5 15 </Coords>
                                                                </Polygon>
                                                        </SpatialLocator>
                                                        <Text xml:lang="en"> 
Sea </Text>
                                                </StillRegion>
                                                <StillRegion id="2">
                                                        <SpatialLocator>
                                                                <!-- more 
elements here -->
                                                        </SpatialLocator>
                                                        <Text xml:lang="en"> 
Oacen </Text>
                                                </StillRegion>

                                                <!-- more Still Regions here 
here -->

                                        </SpatialDecomposition>
                                </Image>
                        </Content>
                </Collection>
        </MultimediaContent>
</Description>

</Mpeg7>






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