code: public function uniqueKey(array $colNames, /*# string */ $extraClause = '')
      {
          if (!isset($this->tbl_constraint['unique'])) {
              $this->tbl_constraint['unique'] = [];
          }
          $this->tbl_constraint['unique'][] = [$colNames, $extraClause];
          return $this;
      }
docs: {@inheritDoc}