# Extension attributes (XSLT only)

An extension attribute is an extra attribute on an XSLT-defined element. These attributes are all in the Saxon namespace `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

- saxon:threads: enables parallel processing of xsl:for-each instructions

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