The affix yat comes in the sense of 'excellent with regard thereto' after the word sabhā ,
This debars यत् the difference being in accent (III. I. 3), and (VI. I. 185) Thus सभायां साधुः = सभ्यः꣡ 'refined' lit. fit for society.,
सभायाः यः
Adhyāya 4 · Pāda 4 · Rule 105