[Zope] turning off inheritance

I am working on generating a directory from Zope objects. There is a folder full of units (other folders). These units contain people (more folders). Units and people both have contact information stored in their properties. How do I detect if a person has, for example, 'contact_email' set? I've tried <dtml-if "_.hasattr(this(), 'contact_email')"> It succeeds if the person doesn't have contact_email set but its unit does. I've also tried variations of <dtml-with ... only>, with no success. Any pointers? Where could I have found the answer to this on my own? Thank you. --kyler