On grammatical class:

fe'i
n-ary mathematical operator: divided by; division operator; [(((a / b) / c) / ...)].
pi'i
n-ary mathematical operator: times; multiplication operator; [(((a * b) * c) * ...)].
su'i
n-ary mathematical operator: plus; addition operator; [(((a + b) + c) + ...)].
vu'u
n-ary mathematical operator: minus; subtraction operator; [(((a - b) - c) - ...)].