Add-attribute-020 (AB)

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:add-attribute match="/" attribute-value="5" attribute-name="att">
      <p:with-input>
         <doc attribute="3">
                  Some text
                  <!-- a comment -->
                  <?pi?>
               </doc>
      </p:with-input>
   </p:add-attribute>
</p:declare-step>