The ṣ is optionally substituted in the chandas (Vedas) after the upasarga (Prepositions) ni , vi and abhi when the augment aṭ intervenes.,
Thus न्यषीदत् or न्यसीदत् पिता नः; व्यषीदत् or व्यसीदत् पिता नः, अभ्यषीदत् or अभ्यसीदत् ॥
The anuvritti of सद and स्वंज is not to be read into this sutra. It is, in fact, a general rule and applies to verbs other than these two. As व्यस्तौत्म् न्यष्टौत्, अभ्यष्टौत or अभ्यस्तौत ॥,