code: def _validate_unique(self, unique, field, value):
          
          if unique:
              root_key = self.schema_path[0]
              data = (doc[field] for doc in self.root_document[root_key])
              for key, count in collections.Counter(data).items():
                  if count > 1:
                      msg = "'{}' is not unique".format(key)
                      self._error(field, msg)
docs: Ensure value uniqueness.
  
          The rule's arguments are validated against this schema:
          {'type': 'boolean'}