# Extension attributes (XSLT only)

An extension attribute is an extra attribute on an XSLT-defined element. Following the rules of XSLT, such attributes must be in a non-default namespace. For Saxon extension elements, the namespace must be the Saxon namespace URI "http://saxon.sf.net/"

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

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