Access to base URIs

Consider this pipeline:

<p:pipeline name="pipeline"
	    xmlns:p="http://www.w3.org/2007/03/xproc"
	    xmlns:nw="http://nwalsh.com/xproc/pipelines">
<p:input port="source">
  <p:document href="xproc.xml"/>
</p:input>

<p:import href="library.xml"/>

<nw:do-some-stuff/>

<p:store name="store">
  <p:option name="href" value="???"/>
</p:store>

</p:pipeline>

Suppose I want to set the href value on the store option to the base URI
of the pipeline's source input with .xml changed to .html.

That's a use case we need to support, right?

How do I do that?

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | Four things come not back: the spoken
http://nwalsh.com/            | word, the sped arrow, time past, the
                              | neglected opportunity.--Omar Ibnal-Halif