अतो भिस ऐस्

Karika

एत्वं भिसि परत्वाश्चेदत ऐस्क्व भविष्यति ।
कृतेप्येत्वे भौतपूर्व्यादैस्तु नित्यस्तथा सति ॥

As वृक्षैः, प्लक्षैः, अतिजरसैः ॥ Why do we say ending in अ? Observe अग्निभिः, वायुभिः ॥ Why 'short अ'? Observe खट्वाभिः, मालाभिः ॥ The adhikara of \अतः\ ('after a short अ'), extends up to 7.1.17.

The form अतिजरसैः illustrates some important principles of grammar. अति + जरा = (जरामतिक्रान्तः) अतिजर (अ being shortened by 1.2.48). The word अतिजर ends in अ, and there fore forms its Instrumental plural by ऐस् ॥ Thus अतिजर + ऐस् ॥ Now we apply 7.2.101, which says for जरा is substituted जरस् before vowel-beginning case-endings. It should not be objected, that the substitution is ordained for जरा and not जर; for the maxim एकदेशविकृतमनन्यवद् भवति (a tail-less dog is still a dog) applies here. Having made this substitution, we get अतिजरसैः ॥ Nor should you object that it was the अ of जर which had given occasion to the existence of ऐस्, and that अ should not be destroyed, on the maxim संनिपातलक्षणो विधिरनिमित्तं तद् विघातस्य, for this maxim is not universal, as Panini himself shows in employing the form कष्टाय (III. I. 14), in which ए of the Dative is changed into य by 7.1.13, and then this very य causes the destruction of अ and makes it आ (See 7.1.13 also).

Karika:- If there be substitution of ए for the final अ before the affix भिस् by the subsequent sutra 7.3.103, where will then the present sutra, causing the substitution of ऐस् for भिस् after stems ending in अ, find its scope, (because there will be no stem left ending in अ)? If even after changing अ into ए, you change the भिस् into ऐस्, because the ए was once अ (भौतपूर्व्यात्); then the rule of changing भिस् into ऐस् becomes a nitya rule, because it takes effect even after the application of rule 7.3.103, and being nitya it debars that rule. Thus वृक्ष + भिस् ॥ Here the subsequent rule 7.3.103, requires the अ to be changed into ए ॥ Thus वृक्षे + भिः Now the present rule cannot apply because there is no अदन्त stem. However, is considering that वृक्षे once was अदन्त, we change भिः to ऐस् then the rule becomes a nitya rule. In fact, it is a nitya rule and debars the application of 7.3.103, within its jurisdiction, 7.3.103, finds its scope in वृक्ष + सु = वृक्षेषु ॥,