Namespace in the XPath tag

Hi,
in the current XML DSIG specification [1], under
section 6.6.3, there is an example of how an XPath tag
will look like inside a <Signature> tag. I've noticed
an additional namespace declaration in the XPath tag:
<XPath xmlns:dsig="&dsig;">.

Just want to clarify whether this is mandatory if my
XPath expression does not involve any Signature tags
or tags belonging to the Signature namespace.

Cheers,
Herry

[start example below from [1]]

 <Document>
   ...   
   <Signature
xmlns="http://www.w3.org/2000/09/xmldsig#">
     <SignedInfo>
      ...
       <Reference URI="">
         <Transforms>
           <Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
             <XPath xmlns:dsig="&dsig;">
             not(ancestor-or-self::dsig:Signature)
             </XPath>
           </Transform>
         </Transforms>
         <DigestMethod
Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
         <DigestValue></DigestValue>
       </Reference>
     </SignedInfo>
     <SignatureValue></SignatureValue>
    </Signature>
    ...
   </Document>

[1] http://www.w3.org/TR/xmldsig-core/

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