[Zope-dev] manage_editProperties??

I'd like to change my manage_editProperties method of my property sheets to maintain the currently set value of a variable if one doesn't exist in the REQUEST object...if one IS there, but is None, then I can wipe it out... The problem I'm having is wanting to update a single property value, without specifying all the values. I just want to increment one value and set it again...but when I do, it wipes all the other variables out because I don't set them in the REQUEST object... Is there an easy way to do this?? Thanks! Jon Brisbin