The affixes ḍyat and ḍya come in the sense of 'what stays there' after the word srotas and before these affixes the final syllable as of srotas is elided, in the chandas (Vedas).,
This debars यत् which comes in the alternative. As स्रोतसि भवः = स्रोत्यः॑ or स्रोत्यः॒॒꣡ the difference being in the accent 3.1.3 and 6.1.185. The anubandha ड causes the elision of अस् of स्रोतस् ॥ When यत् is added the form is स्रोतस्यः॑ ॥,