d or ru may optionally be substituted for the s of a root, , before the personal ending si (s) when such s stands at the end of a word.,
Thus अचकास् त्वम् or अचकात् त्वम्, अन्वशास् त्वम् or अन्वशात् त्वम् ॥ For the रु is first substituted visarjaniya, which is then changed to स् ॥
The word धातुः is employed in the sutra for the sake of the subsequent sutras : so also the word रु ॥,