public class Double3Accum : IThreeSourcesHandler, IDoubleReturns, IStreamHandler, IDoubleInputs {
  public IList<double> Execute(IList<double> source1, IList<double> source2, IList<double> source3) {
    int count = source1.Count;
    var values = new double[count];
    for (int i = 0; i < count; i++) {
      values[i] = source1[i] + source2[i] + source3[i];
    }
    return values;
  }
}