xpath question

[Anamitra . Bhattacharyya]

Hi All
I have an xml which looks something like this below. I am currently in the
invoice node and I want to select all the line nodes that have the same
invoicenum as the invoice node that I am in. It will be almost like this:

../line[invoicenum= <the current invoice elements invoicenum>]

I am not sure how to express that <the current invoice elements invoicenum>
in this xpath expr. Can any one suggest something.

<Root>
    <invoice>
        <invoicenum>aaaa</invoicenum>
    </invoice>
    <invoice>
        <invoicenum>bbbb</invoicenum>
    </invoice>
    <line>
         <desc>line1 for aaaa</desc>
         <invoicenum>aaaa</invoicenum>
    </line>
    <line>
         <desc>line2 for aaaa</desc>
         <invoicenum>aaaa</invoicenum>
    </line>
    <line>
         <desc>line1 for bbbb</desc>
         <invoicenum>bbbb</invoicenum>
    </line>
</Root>

thanks
Anamitra