[Zope] Very lightweight form validation

I need to do some minimal validation on a few forms. The built-in typechecking (http://www.zope.org/Members/Zen/howto/FormVariableTypes) does about 99% of what I need, with the exception of outputting useful error messages. I've looked at some of the various form-processing add-ons but all of them see very "heavy" when what I mainly want to do is ensure that the user remembered to fill in their last name, and if not, tell them that they need to enter it and try again. Seriously, that's about the sophistication level I'm targeting. Any suggestions? -- Kirk Strauser The Day Companies