शॄदॄप्रां ह्रस्वो वा

In 1. शॄ 'to injure' 2. दॄ 'to tear' and 3. पॄ 'to protect'-- all belonging to क्र्यादिगणः (9th Conjugation), the vowel may be optionally shortened in the लिट् (Perfect Tense).
As वि शश्रतुः by shortening or वि शशरतुः, by guna, which prevents ए and abhyasalopa 6.4.126 वि शश्रुः or विशशरुः, विदद्रतुः or विददरतुः, विदद्रुः or विददरुः; निपप्रतुः or नि पपरतुः, नि पप्तुः or नि पपरुः ॥ Why is the word 'short' made optional in the sutra, and not the word 'guna'; for in the absence of guna, the long ॠ would have become र before अतुः by यणादेशः, and we would have got the forms विशश्रतुः, विशश्रुः by the regular rules of 'sandhi'? The word 'short' is used in the sutra, in order to debar the इर् and उर् alternatives. Had the sutra been "शॄ दॄ प्रां वा" the alternative examples would have been with इर् 7.1.101 as शिशिरतुः दिदिरतुः and उर् 8.2.77 as पिपुरतुः ॥ Some say this sutra is unnecessary. The above forms like वि शश्रतुः, विशश्रुः, &c. can be regularly obtained from the roots श्रा 'to cook', द्रा 'to abuse', and प्रा 'to fill' and as the roots have many other senses, besides those assigned to them in the Dhatupatha, the roots श्रा, द्रा and प्रा will give the meanings of शॄ, दॄ and पॄ ॥ If that were so, the form विशशृवान् with the affix क्वसु could not be formed, for we should have either विशशर्वान् from शॄ root or विशश्रिवान् from श्रा root but never विशशृवान् ॥ So the rule about shortening is necessary.