code: def validate(self, value):
          
          try:
              # trap blank fields here
              if not self.blank or value:
                  v = int(value)
                  if v < 0:
                      return None
              return value
          except ValueError:
              return None
docs: Applies the validation criteria.
          Returns value, new value, or None if invalid.
  
          Overload this in derived classes.