docs: def get_default_prefix(self, instance=None):
          """Gets the prefix for this form.
  
          :param instance: the form model instance.  When calling this method
              directly this should almost always stay None so it looks for
              self.instance.
          """
          if instance is None and hasattr(self, 'instance'):
              instance = self.instance
  
          if instance and instance.id is not None:
              # it's an existing instance, use the instance prefix
              instance_prefix = self.default_instance_prefix
              if instance_prefix is None:
                  instance_prefix = self.__class__.__name__.lower() + 'i-'
  
              return '{0}{1}'.format(instance_prefix,
                                     instance.id)
  
          if self.default_new_prefix is not None:
              return self.default_new_prefix
  
          return self.__class__.__name__.lower() + 'new-'