public function add($type)
    {
        if (!is_string($type)) {
            throw new \InvalidArgumentException(
                'A type should be represented by a string, received: '
                .var_export($type, true)
            );
        }

        // separate the type by the OR operator
        $type_parts = explode(self::OPERATOR_OR, $type);
        foreach ($type_parts as $part) {
            $expanded_type = $this->expand($part);
            if ($expanded_type) {
                $this[] = $expanded_type;
            }
        }
    }