Praudhamanorama p128

p128

"iṇaḥ ṣaḥ" (Pā. Sū. 8.3.39): This is an exception to the preceding rule. Both this sūtra and "saḥ" continue into subsequent rules. The distinction is: according to possibility, ṣa occurs after iṇ, and sa occurs after anything else.
In "puraḥ praveṣṭavyāḥ": from pṝ meaning protection or filling, kvip occurs by "bhrājabhāsa—" (Pā. Sū. 3.2.177) etc. After that ending in that [kvip], jas occurs. To make clear that it ends in jas, "pūḥ purau" is cited.