saxon:trace on the xsl:accumulator element causes a trace message to be
output (to the Configuration's Logger) whenever the value of the accumulator changes. The
message includes the name of the accumulator, the path of the node, and the new accumulator
Since 9.9.1, the attribute can also be used on the xsl:mode element. It causes tracing (messages output to the
Configuration's Logger) of all template rules executed in the mode, showing the nodes selected by
xsl:apply-templates and the rules used to process them.