Merge two accumulated values together.
Merge two accumulated values together. Is allowed to modify and return the first value for efficiency (to avoid allocating objects).
one set of accumulated data
another set of accumulated data
both data sets merged together
Return the "zero" (identity) value for an accumulator type, given its initial value.
Return the "zero" (identity) value for an accumulator type, given its initial value. For example, if R was a vector of N dimensions, this would return a vector of N zeroes.