grouping assignment
> 
> I tried this before, but I got the error: A sequence of more 
> than one item is not allowed as the first argument of 
> saxon:evaluate(). 

Sorry, it should of course be

> group-by="string-join(receiver, operation, prev_bus_state, data,
>   for $x in $groupingKeys/key return saxon:evaluate($x), '|'))"
> 
> 
Michael Kay
http://www.saxonica.com/