docs: def _validate_condition_keys(self, field, value, error):
          """
          Validates that all of the keys in one of the sets of keys are defined
          as keys of ``value``.
          """
          if 'field' in value:
              operators = self.nonscalar_conditions + self.scalar_conditions
              matches = sum(1 for k in operators if k in value)
              if matches == 0:
                  error(field, 'Must contain one of {}'.format(operators))
                  return False
              elif matches > 1:
                  error(
                      field,
                      'Must contain no more than one of {}'.format(operators)
                  )
                  return False
              return True
          elif 'and' in value:
              for condition in value['and']:
                  self._validate_condition_keys(field, condition, error)
          elif 'or' in value:
              for condition in value['or']:
                  self._validate_condition_keys(field, condition, error)
          else:
              error(field, "Must contain field + operator keys, 'and', or 'or'.")
              return False