code: private function normalizeComplexFilter(array $filter)
      {
          $result = [];
          foreach ($filter as $key => $value) {
              if (isset($this->filterControls[$key])) {
                  $key = $this->filterControls[$key];
              } elseif (isset($this->attributeMap[$key])) {
                  $key = $this->attributeMap[$key];
              }
              if (is_array($value)) {
                  $result[$key] = $this->normalizeComplexFilter($value);
              } else {
                  $result[$key] = $value;
              }
          }
          return $result;
      }
docs: Normalizes complex filter recursively.
  @param array $filter raw filter.
  @return array normalized filter.