public function process()
    {
        $operands = $this->getOperands();

        if ($operands->containsNull() === true) {
            return null;
        }

        if ($operands->exclusivelySingle() === false) {
            $msg = "The Substring operator only accepts operands with a single cardinality.";
            throw new OperatorProcessingException($msg, $this, OperatorProcessingException::WRONG_CARDINALITY);
        }

        if ($operands->exclusivelyString() === false) {
            $msg = "The Substring operator only accepts operands with a string baseType.";
            throw new OperatorProcessingException($msg, $this, OperatorProcessingException::WRONG_BASETYPE);
        }

        $operand1 = $operands[0];
        $operand2 = $operands[1];

        $call = ($this->getExpression()->isCaseSensitive() === true) ? 'mb_strpos' : 'mb_stripos';

        return new QtiBoolean($call($operand2->getValue(), $operand1->getValue(), 0, 'UTF-8') !== false);
    }