Integration

Hello
I wonder if fn:trace or something familiar can be used on Oracle Service Bus XQuery transformations.
I'd like to simply debug/trace some own variables.
For example, let't think of a function in a osb xquery transformation:

declare function local:removeSomething($inString xs:string) as xs:string
{
let $firstPart:= fn:substring($inString,1,4)
let $secondPart:= fn:substring($inString,5,2)
return $secondPart
};

I mean this is a simple sample with no sense, but I'd like to know what's set on runtime in my $firstPart variable.
How can I simply get informations about my variable values?
Couldn't I just use fn:trace($firstPart, 'The value of $firstPart is: ')
But where will this trace information be written to? Log? Console?

Any help would be appreciated.
Thanks
Best regards