Conditionally adding patternset to fileset

Question

I want to add an exclusion list, if and only if the paternset actually exists. I wasn't sure how to check if a patternset exists, so I added a regular property to serve as a flag.

<echo message="${property::exists('package.processSpecials')}" />
<fileset id="subprojects" basedir="${build.workspace}" >
    <include name="*" />
    <patternset refid="package.excludespecials" if="package.processSpecials" />
</fileset>

This fails because package.excludespecials does not exist, even though package.processSpecials does not exist.

[echo] False

BUILD FAILED

...package.build(288,10):
patternset reference 'package.excludespecials' not defined.

I tried putting an if statement inside the fileset definition, but it was an unexpected element.

My other alternative that feels a bit clunky is:

<if test="${property::exists('package.processSpecials')}">
    <fileset id="subprojects" basedir="${build.workspace}" >
        <include name="*" />
        <patternset refid="package.excludespecials" />
    </fileset>
</if>

<if test="${not property::exists('package.processSpecials')}">
    <fileset id="subprojects" basedir="${build.workspace}" >
        <include name="*" />
    </fileset>
</if>