saxon:asynchronous
This attribute may be set on the xsl:result-document instruction. The default for Saxon-EE is
       "yes", which causes the instruction to be evaluated in a separate thread, if there is a spare thread available (the
       maximum number of threads used is configurable using FeatureKeys.RESULT_DOCUMENT_THREADS).
A possible reason for setting the value to "no" is that the stylesheet calls extension functions with side-effects, where it is important for the calls to be in the right order.
Asynchronous processing of xsl:result-document is automatically suppressed if
         tracing (using a TraceListener) is enabled.