This attribute may be set on the xsl:function element. The permitted values
are "yes" and "no". Specifying "yes" indicates that Saxon should remember the
results of calling the function in a cache, and if the function is called again
with the same arguments, the result is retrieved from the cache rather than being
recalculated. Don't use this if the function has side-effects (for example, if
it calls saxon:assign, or an extension function with side-effects).
Don't use it if the function accesses context information such as the context node
or position() or last(). And be careful if the function constructs and returns a
temporary tree: the effect will be that the function returns the same tree each
time, rather than a copy of the tree (this difference will only show up if you
compare the identity of nodes in the two trees).
Memo functions are not available in Saxon-HE. When running under Saxon-HE, this attribute is ignored with a warning.
