This attribute may be set on the
xsl:copy-of element. The allowed values are
"yes" and "no". If set to yes, this attribute enables the optimization
described in Serial Processing of Large Documents.
The attribute should be set to "yes" only if any external document read by this instruction is read at most
once by the stylesheet. If this is not the case, the optimization is unsafe, since the functions
document() are required to return the same results each time they are called.
This optimization is available only with Saxon-SA.