Sum on child nodes based on attribute value in xslt

Question

Hi all,
Any one can post helpful code to calculate the sum of child nodes based on the attribute value of other child node.
Let's say for example.
I have one child element has attribute value let's say Tax so I have to put condition on child element having value of "Tax" I need to calcualte sum of Invoice amount that element is also child node
After calculating sum I have to apply that sum at the header level of the Invoice. For each Invoice I have calculate sum and apply at the header level.

Answer 1

It isn't working :(.Got the following error:
An error was reported compiling the XPath expression: error: XPath expression invalid, not a selection: declare namespace ws = 'http://www.bea.com/wli/sb/transports/ws';
declare namespace tuxedo = 'http://www.bea.com/wli/sb/transports/tuxedo';
declare namespace wsa = 'http://schemas.xmlsoap.org/ws/2004/08/addressing';
declare namespace http = 'http://www.bea.com/wli/sb/transports/http';
declare namespace xsi = 'http://www.w3.org/2001/XMLSchema-instance';
declare namespace wsp = 'http://schemas.xmlsoap.org/ws/2004/09/policy';
declare namespace file = 'http://www.bea.com/wli/sb/transports/file';
declare namespace xsd = 'http://www.w3.org/2001/XMLSchema';
declare namespace soap12-env = 'http://www.w3.org/2003/05/soap-envelope';
declare namespace flow = 'http://www.bea.com/alsb/flow/transport';
declare namespace sftp = 'http://www.bea.com/wli/sb/transports/sftp';
declare namespace sb = 'http://www.bea.com/wli/sb/transports/sb';
declare namespace soap-enc = 'http://schemas.xmlsoap.org/soap/encoding/';
declare namespace ejb = 'http://www.bea.com/wli/sb/transports/ejb';
declare namespace soap-env = 'http://schemas.xmlsoap.org/soap/envelope/';
declare namespace jpd = 'http://www.bea.com/wli/sb/transports/jpd';
declare namespace email = 'http://www.bea.com/wli/sb/transports/email';
declare namespace tp = 'http://www.bea.com/wli/sb/transports';
declare namespace dsp = 'http://www.bea.com/dsp/transport/sb';
declare namespace ctx = 'http://www.bea.com/wli/sb/context';
declare namespace soap12-enc = 'http://www.w3.org/2003/05/soap-encoding';
declare namespace wsu = 'http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd';
declare namespace jms = 'http://www.bea.com/wli/sb/transports/jms';
declare namespace ftp = 'http://www.bea.com/wli/sb/transports/ftp';
fn:string(./xml-fragment/@IntObjectName).