The affix yat comes in the sense of 'excellent in regard thereto' after a word in the Locative -7th case in construction.,
Thus सामसु साधुः = सामन्यः॑ 6.4.168 'conversant with the Sama-Veda'. So also वेमन्यः, कर्मण्यः, शरन्यः &c. The word साधु here means प्रवीण, or योग्य 'expert' and 'fit', and does not mean उपकारक 'a benefactor or a good person'. When the sense is that of 'good', the sutra तस्मै हितम् (V. I. 5) will apply.,