/**
  * @see http://php.net/manual/fr/class.arrayaccess.php
  */
 public function offsetSet($offset, $value)
 {
     if (!isset($this->flip[$offset])) {
         throw new RuntimeException("Can not modify the value corresponding to this key, as it is not defined in structure");
     }
     return parent::offsetSet($offset, $value);
 }