public function setPosition($position)
    {
        switch ($position) {
            case Modification::POSITION_ANY:
            case Modification::POSITION_NTERM:
            case Modification::POSITION_CTERM:
            case Modification::POSITION_PROTEIN_NTERM:
            case Modification::POSITION_PROTEIN_CTERM:
                $this->position = $position;
                
                break;
            default:
                throw new \InvalidArgumentException('Postion must be any or terminus (see POSITION_XXXX)');
        }
    }