The Fontconfig documentation is fairly opaque. I want applications looking up the 'Helvetica' family to instead receive the 'Helvetica LT Std' family and display that font, browsers in particular. The latest XML file I wrote to ~/.fontconfig/fonts.conf was
<fontconfig>
<match target="pattern">
<test name="family" target="default" compare="eq">
<string>Helvetica</string>
</test>
<edit name="family" mode="assign">
<string>Helvetica LT Std</string>
</edit>
</match>
</fontconfig>
What am I doing wrong and what would be a correct way to express this substitution?