deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
So root /aJcu loses its n न्, becoming /ac''' —
/pra + /aJcu m + /su
→ /pra + /ac''' m + /su by aniditAMh...
→ pra प्र + /aJcu + /kvin + /su by RtvigdadhRksr..., because /aJcu is ifc here
→ pra प्र + ac अच् + /su by aniditAMh... because /kvin is /kGit
→ prAc प्राच् + /su by akassavarN...
→ prAnc प्रान्च् + /su with /num before strong /su by ugidacAMs..., because /aJcu is an /udit root
→ prAnc प्रान्च् by halGyAbbhyodIrgh...
→ prAn प्रान् by saMyogAntasy...
→ prAG प्राङ् by kvinpratyayasy...
→ !**prAG प्राङ् "eastern dude"
/pra + (/ac''' m + /su)
→ /pra + **aG अङ्
→ **prAG प्राङ् "eastern dude"
/prati + (/ac''' m + /su)
→ /prati + **aG अङ्
→ !**pratyaG प्रत्यङ् "western dude"
Above, you said prAnc प्रान्च् → prAn प्रान् → prAG प्राङ्. Shouldn't prAnc प्रान्च् change into prAJc प्राञ्च् by monusvAraH and anusvArasy... before saMyogAntasy... can work?
No clue, sorry. That depends on what rules work first. The /pANinIya always prescribes a definite order of operations, but I sometimes screw up. Anyway, if Thou Arst Write and prAJc प्राञ्च् works before saMyogAntasy..., then kvinpratyayasy... still turns the J ञ् into G ङ्. Your homework: read the commentarists and find out if STunASTuH works here or not.
So what happens when this /pra + /ac''' m is not before a strong?
Then there is no n न्, obviously.
When whimpy —
**prAc- प्राच् + /Gas → !**prAcas प्राचस्
**prAc- प्राच् + /Am → !**prAcAm प्राचाम्
When /pada —
**prAc- प्राच् + /sup' → **prAk- प्राक् + /su → !**prAkSu प्राक्षु with /Satvam
Why do we say "unless /idit"?
The root nand नन्द् is an /idit root (see iditonumdh...), so it never loses its nexttolast n न् —