Hmm. Unless it's something like:

if $x = "" { $x = $y } else { $x = $z }

...or something like that. I'm thinking back to the "leave blank for default" discussion elsewhere (per the wizard), and that if the user left the entry blank, then the default would prevail. In which case, the input -- $x -- would be = "" instead of = "user-input".

*ponders*