void testTree() {
  ROOT::RDataFrame d(10);
  int i(1);
  vector<string> definedNames = {"b1", "b2", "b3"};
  
  d.Define("b1", [&i]() { int j = i; ++i; return j; })
    .Define("b2", [&i]() {
        vector<float> v(i);
        for (auto &k:v)
          k = gRandom->Rndm();
        return v;
      })
     .Define("b3", [&i]() {
        vector< vector<float> > vv;
        for (auto &v1:vv) {
          v1.resize(i);
          for (auto &k:v1)
            k = gRandom->Rndm();
        }
        return vv;
      })
    .Snapshot("tr", "testTree.root", definedNames);
}