This attribute may be set on a global xsl:variable element. The permitted values
are "yes" and "no". If the variable is the subject of a
saxon:assign instruction, it must be set to the value "yes".
Setting this value to "yes" also ensures that the variable is actually evaluated, which is
useful if the select expression calls extension functions with side-effects;
without this, a variable that is never referenced may never be evaluated.
