subInt64 example

    public static function fe_mul121666(ParagonIE_Sodium_Core32_Curve25519_Fe $f)
    {
        /** @var array<int, ParagonIE_Sodium_Core32_Int64> $h */
        $h = array();
        for ($i = 0; $i < 10; ++$i) {
            $h[$i] = $f[$i]->toInt64()->mulInt(121666, 17);
        }

        $carry9 = $h[9]->addInt(1 << 24)->shiftRight(25);
        $h[0] = $h[0]->addInt64($carry9->mulInt(19, 5));
        $h[9] = $h[9]->subInt64($carry9->shiftLeft(25));

        $carry1 = $h[1]->addInt(1 << 24)->shiftRight(25);
        $h[2] = $h[2]->addInt64($carry1);
        $h[1] = $h[1]->subInt64($carry1->shiftLeft(25));

        $carry3 = $h[3]->addInt(1 << 24)->shiftRight(25);
        $h[4] = $h[4]->addInt64($carry3);
        $h[3] = $h[3]->subInt64($carry3->shiftLeft(25));

        $carry5 = $h[5]->addInt(1 << 24)->shiftRight(25);
        $h[6] = $h[6]->addInt64($carry5);
        

->addInt64($f[4])->shiftRight(26)
            ->addInt64($f[5])->shiftRight(25)
            ->addInt64($f[6])->shiftRight(26)
            ->addInt64($f[7])->shiftRight(25)
            ->addInt64($f[8])->shiftRight(26)
            ->addInt64($f[9])->shiftRight(25);

        $f[0] = $f[0]->addInt64($q->mulInt(19, 5));

        $carry0 = $f[0]->shiftRight(26);
        $f[1] = $f[1]->addInt64($carry0);
        $f[0] = $f[0]->subInt64($carry0->shiftLeft(26));

        $carry1 = $f[1]->shiftRight(25);
        $f[2] = $f[2]->addInt64($carry1);
        $f[1] = $f[1]->subInt64($carry1->shiftLeft(25));

        $carry2 = $f[2]->shiftRight(26);
        $f[3] = $f[3]->addInt64($carry2);
        $f[2] = $f[2]->subInt64($carry2->shiftLeft(26));

        $carry3 = $f[3]->shiftRight(25);
        $f[4] = $f[4]->addInt64($carry3);