code: public function withKey($key)
      {
          $keys = func_num_args() > 1 ? func_get_args() : (array)$key;
  
          foreach ($keys as $key) {
              $this->keys[] = $key;
          }
  
          return $this;
      }
docs: Adds a key or many keys (columns) to the result.
  Pass an array of keys or just one key or multiple (string) arguments
  
  @param array|string $key
  @return self