[Zope] How do you check for null search fields?

[Michael]

I have a search form with 3 different fields and need to verify that there is 
a value in at least one of them.  I know that <dtml-if expr="some_field"> 
will test for one, but how do I determine if anyone of the three contains a 
value?  I've checked the how-to's and Google but can't find anything.  I'm 
using ZODB.

TIA,

Michael