mageekguy\atoum\asserters\dateTime::isImmutable PHP Method

    public function isImmutable($failMessage = null)
    {
        if (version_compare(PHP_VERSION, '5.5.0') < 0 || $this->valueIsSet()->value instanceof \dateTimeImmutable === false) {
            $this->fail($failMessage ?: $this->_('%s is not immutable', $this));
        } else {
            $this->pass();
        }
        return $this;
    }