{

// source vector

const RVector<double> input;

// accumulated value

double value;

// constructors

Sum(const NumericVector input) : input(input), value(0) {}

Sum(const Sum& sum, Split) : input(sum.input), value(0) {}

// accumulate just the element of the range I've been asked to

void operator()(std::size_t begin, std::size_t end) {

value += std::accumulate(input.begin() + begin, input.begin() + end, 0.0);

}

// join my value with that of another Sum

void join(const Sum& rhs) {

value += rhs.value;

}

// declare the SumBody instance

Sum sum(x);

// call parallel_reduce to start the work

parallelReduce(0, x.length(), sum);

// return the computed sum

return sum.value;