स्वमोर्नपुंसकात्

The Nominative 1st-Case and Accusative -2nd case endings सु and अम् are elided after a neutral अङ्ग (stem).
As दधि तिष्ठति, दधि पश्य, मधु तिष्ठति, मधु पश्य ॥ So also त्रपु and जतु ॥ In तद् ब्राह्मणकुलम् the word तद् has lost its nominative singular ending by this rule, which has thus superseded the subsequent rule 7.2.102, by anticipation, by which अ replaces the final द् of तद् ॥ Or this luk rule of the present sutra is a Nitya rule compared with 7.2.102. How can this be a nitya rule when it is set aside by the next rule अतोऽम् ॥ We still call this rule nitya, on the maxim यस्य च लक्षणान्तरेण निमित्तं विहन्यते न तदनित्यम् "(an operation) the cause of which would, (after the taking effect of another operation that applies simultaneously), be removed by another (third rule), is not, (on that account regarded as) not nitya". For here the cause which is luk-elision, is removed by the following aphorism अतोऽम् which ordains an अम् instead of luk, and not by 7.2.102. For the application of rule 7.2.102, the necessary condition is that a case-affix should follow तद् &c. But when the case-affix itself is luk-elided, the substitution taught in that sutra cannot take place.