The Genitive 6th-Case-affix is optionally elided after a stem ending in ṛ when 1. svasṛ and 2. pitṛ follow.,
Thus मातुःस्वसा or मातुःष्वसा or मातृष्वसा, पितुःस्वसा or पितुःष्वसा or पितृष्वसा ॥ When the affix is elided, then स is invariably changed to ष by 8.3.85; and when it is not elided, then the change is optional 8.3.85. So also दुहितुः पतिः or दुहितृपतिः, ननान्दुः पतिः or ननान्दृपतिः ॥,