There is an "a" that occurs in Crow between a main verb and a conjoined
continuative auxiliary, e.g.: huu-a-lawi'-k
come-A-continue-DECL
'he kept coming, he was coming along'
It also shows up in Hidatsa, and Mandan has a ha: 'simultaneous'. Could
these be related to uN? It's hard to know when you are dealing with such
short morphemes.
Randy