fun f(aggList: List<Pair<Long, Long>?>): Pair<Long, Long>? (source)
Function's component for the combination of aggregate values.
E.g.: in the case of arithmetic mean, a1,a2 = <c1,s1>, <c2,s2> (the count and the sum of each aggregate value) f(a1,a2) = <(c1+c2),(s1+s2)> (the combined aggregate value).
aggList - the list of aggregate values to combine.
Return
the combined aggregate value.