pHoim-no- "foam"

From: Joao
Message: 28615
Date: 2003-12-19

The PIE *pHoim- (English foam, Latin pu:mex, spu:ma) has -n- forms in Sanskrit phe:nas, Lit spaine, Russian pena. What's the explanation for this aparent m/n shift?
a) dissimilation p-m > p-n ?
b) pHoim-no- ?
c) different suffix ?
 
Joao SL