code: def validate(self, data):
          
          if 'verb' in data and data['verb'] != self.__class__.__name__:
              raise ValidationError(
                  # FIXME encode data
                  'This is not a valid OAI-PMH verb:{0}'.format(data['verb']),
                  field_names=['verb'],
              )
  
          if 'from_' in data and 'until' in data and \
                  data['from_'] > data['until']:
              raise ValidationError('Date "from" must be before "until".')
  
          extra = set(request.values.keys()) - set([
              f.load_from or f.name for f in self.fields.values()
          ])
          if extra:
              raise ValidationError('You have passed too many arguments.')
docs: Check range between dates under keys ``from_`` and ``until``.