The pipeline
<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:identity name="producer">
<p:with-input>
<doc a="42"/>
</p:with-input>
</p:identity>
<p:sink/>
<p:add-attribute attribute-name="att">
<p:with-input>
<result/>
</p:with-input>
<p:with-option name="attribute-value" select="/doc/@a">
<p:pipe step="producer1"/>
</p:with-option>
</p:add-attribute>
</p:declare-step>
