अतो लोपः

The अ standing at the end of a अङ्ग (stem) is elided before an आर्धधातुक affix.
Thus चिकीर्षिता, चिकीर्षितुम्, and चिकीर्षितव्यम्, from the Desiderative stem चिकीर्ष ॥ So also धिनुतः and कृणुतः from the roots धिन्व् and कृण्व् thus, धिन्व् + उ 3.1.80 = धिन् + अ+ उ 3.1.80 = धिन् + उ (अ being elided before the ardhatuka उ) = धिनु, the 3rd Personal dual of it is धिनुतः ॥ The addition of अ by 3.1.80 and its subsequent elision by the present sutra, may appear a redundancy, but the elided अ being sthanivat, prevents guna of धि ॥ So also कृणुतः ॥ See sutra 3.1.80. Why do we say "the अ is elided"? Observe चेता, स्रोता here इ and उ have not been elided. Why do we say "अतः with a त्"? The long आ will not be elided : as याता, वाता ॥ Why do we say "before an Ardhadhatuka"? Before a Sarvadhatuka there will be no elision of अ nor before a Taddhita : as वृक्षत्वम् and वृक्षता ॥

Vart:- The elision of such अ takes place even to the supersession of the subsequent rules relating to Vriddhi and lengthening. As चिकीर्षकः, जिहीर्षकः, चिकीर्ष्यते and जिहीर्ष्यते ॥