code: protected function getNextBound($key) {
      if ( false === array_key_exists($key, $this->bounds) )
        $this->bounds[$key] = 0;
  
      $this->bounds[$key] = $this->bounds[$key] + 1;
  
      return $this->bounds[$key];
    }
docs: Return next index bound value for the given key (current scope identifier)
  
  @param   string  $key
  @return  integer