public function add(ilAssQuestionSolutionComparisonExpression $expression)
{
$expression->setDb($this->db);
$expression->setQuestionId($this->getQuestionId());
$expression->setSkillBaseId($this->getSkillBaseId());
$expression->setSkillTrefId($this->getSkillTrefId());
$this->expressions[$expression->getOrderIndex()] = $expression;
}