Before the Aorist in aṅ and before an affix beginning with a consonant having an indicatory k or ṅ there is the substitution of ī for the vowel of śās,
Thus अन्वशिषत्, अन्वशिषताम् and अन्वशिषन्; so also शिष्टः (with क्त), शिष्टवान् (with क्तवतु), तौ शिष्टः वयं शिष्मः (with the tense-affixes तः and मः which are ङित् by 1.2.4). The स is changed to ष by 8.3.60. Why do we say 'before the affixes of अ Aorist and consonant affixes'? Observe शासति, शशासतु, शशासुः ॥
Vart:- There is the substitution of इ for the vowel of शास् before the affix क्वि ॥ As आर्यशीः = आर्यान् शास्ति ॥ So also मित्रशीः ॥ The form is thus evolved, शास् + क्वि = शिस् + ० = शिर + ० = शीः (the short इ being lengthened by 8.2.76).
The root शास् is that root which takes अङ् aorist; namely the second Adadi शास् (शासु अनुशिष्टौ), and not the Bhvadi and the first Adadi शास (आङ शासु इच्छायाम्) Therefore not here आशास्ते, आशास्यमानः ॥
Vart:- But before क्विप्, this शास् also is changed, as आशीः, आशिषौ, आशिषः ॥ Or this is an irregular form indicated by the author in the word क्षियाशीः used in 8.2.104.,