The word paścāt is anomalous in the sense of astāti,
The word is formed by changing अपर into पश्च, and adding the affix आति ॥ As अपरस्यां दिशि वसति = पश्चाद्वसति, आगतः, रमणीयम् &c.
Vart:- So also when अपर is preceded by another word denoting direction, it is changed into pascha and ati is added. As दक्षिणपश्चात् उत्तरपश्चात् ॥
Vart:- So also when अर्ध follows such a compound, apara is changed into pascha; as, दक्षिणपश्चार्धः, उत्तरपश्चार्धः ॥
Vart:- Apara is changed into pascha, when followed by ardha, and not preceded by any other word. As, पश्चार्धः ॥,