public function __construct($filename, array $parameters, $nb_keys)
{
Assertion::integer($nb_keys, 'The key set must contain at least one key.');
Assertion::greaterThan($nb_keys, 0, 'The key set must contain at least one key.');
$this->setFilename($filename);
$this->parameters = $parameters;
$this->nb_keys = $nb_keys;
}