An extension attribute is an extra attribute on an XSLT-defined element. These attributes are all in
the Saxon namespace URI "http://saxon.sf.net/". This namespace needs to be declared in a namespace declaration,
but it does not need to be listed in the extension-element-prefixes attribute.
For example, the saxon:assignable attribute can be set as follows:
<xsl:variable name="counter" saxon:assignable="yes"
xmlns:saxon="http://saxon.sf.net/">
The extension attributes provided with the Saxon product are as follows:
saxon:assignable: marks a variable as updateable
saxon:explain: requests display of optimized expression tree
saxon:memo-function: marks a function as a memo function
saxon:read-once: enables serial processing of source documents