paugbu
fu'ivla
x1 (plural) each satisfying x2 (ka), one by one, is how x3 (nu) is brought about.
Defined so that "fi'o paugbu ..." = "one ... at a time". (x3 should describe the "gestalt action" formed by all the individual actions of x2 being done to the x1s one at a time.) See pau'u.