xpath for signing non-closure xml document

Hi All,

I am attempting to make use of xpath expression to ignore xml tags that is 
inserted into a xml document after it has been signed.
I'm using the following XPath expression

not(ancestor-or-self::b/following-sibling::*)

However i can't seem to validate the signature after that.

input xml tag
<a>
<b>
<signature>
.......
</signature>
</b>
<a>

subsequent xml document
<a>
<b>
<signature>
.......
</signature>
</b>
<c>
</c>
<a>


Anyone got any idea?

Rgds and tia.
jernkuan