Dealing with unbound options

Someone please tell me I'm having a brain cramp. I want to write a
recursive-directory-list step. It starts out like this:

<p:declare-step type="cx:recursive-directory-list">
  <p:output port="result"/>
  <p:option name="path" required="true"/>
  <p:option name="include-filter"/>
  <p:option name="exclude-filter"/>
  <p:option name="depth" select="-1"/>
  
  <p:directory-list>
    <p:with-option name="path" select="$path">
      <p:empty/>
    </p:with-option>
    <p:with-option name="include-filter" select="$include-filter">
      <p:empty/>
    </p:with-option>
    <p:with-option name="exclude-filter" select="$exclude-filter">
      <p:empty/>
    </p:with-option>
  </p:directory-list>
</p:declare-step>

Except that the reference to $include-filter is an error if the
include-filter option wasn't specified on the call to
cx:recursive-directory-list.

I think I remember talking about this. Someone please tell me we
thought of a solution.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | When someone tells you something defies
http://nwalsh.com/            | description, you can be pretty sure
                              | he's going to have a go at it
                              | anyway.--Clyde B. Aster