For the non-final t and d of tyad etc. there is substituted s in the Nominative 1st-Case singular.,
As त्यद् + सु = त्य + अ + सु 7.2.102 = स्य + अ + सु 7.2.106 = स्यः 6.1.97. Similarly सः from तद्, एषः from एसद् as एतद् + सु = एत + अ + स् 7.2.102 = एस + अ + स् 7.2.106 = एषः (VI. I. 97). So असौ from अदस् by the following sutra. Why do we say 'non-final'? Observe हे से, सा ॥ Had not this word been used, the case-affix would not be elided in the vocative, as then there would have been no short vowel, as required by 6.1.69.,