using namespace ROOT::VecOps;

RVec<double> VecSum(const RVec<double> &v1, const RVec<double> &v2){
    return v1 + v2;
}

void StandAlone() {
    ROOT::RDataFrame df("Tree", "RDF.root");
    auto Node = df.Define("w", "x * y * rndPoisson");
    auto Sums = Node.Reduce< RVec<double> (const RVec<double> &, const RVec<double> &)>(VecSum, "w", RVec<double>(100, 0));
    cout << Sums.GetValue() << endl;
}