The pipeline
<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" xmlns:xs="http://www.w3.org/2001/XMLSchema" version="3.0">
<p:output port="result"/>
<p:add-attribute match="/" attribute-name="a">
<p:with-input port="source">
<doc/>
</p:with-input>
<p:with-option name="attribute-value" as="foo" select="5"/>
</p:add-attribute>
</p:declare-step>
