// sample reduction function for a sum
T sum_reduce(T a, T b) {
  return (b + a);
}

struct P {
  TAG tag;
  T   value;
};

vector<P> vect_scalar_sum(vector<P> v) {
  map<TAG, T> sum;

  sum.reset();

  for (int i=0; i < v.length(); i++) {
    sum[v[i].tag] = sum_reduce(v[i].value, sum[v[i].tag]);
  }

  vector<P> sum_vect;

  sum_vect.reset();

  for (map<TAG, T>::iterator it = sum.begin();
       it != sum.end(); it++) {
    P tmp;
    tmp.tag = (*it).first; // STL map keeps the tag in a field called first
    tmp.value = (*it).second;  // STL map keeps the value in a field called second
    sum_vect.push_back(tmp);
  }

  return sum_vect;
}