saxon:assignable
This attribute may be set on a global xsl:variable or xsl:param 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.