svoid audio_makeSampleFromSources(L<VF1<double[]>> sources, double[] pair, double gain, VF1<VF1<double[]>> removeSource) {
  double l = 0, r = 0;
  int len = syncSize(sources);
  for (int i = 0; i < len; i++) { // synchro-safe, garbage-free iteration
    VF1<double[]> source = syncGet(sources, i);
    continue if source == null;
    callF(source, pair);
    if (isNaN(pair[0])) continue with callF(removeSource, source);
    l += pair[0];
    r += pair[1];
  }
  pair[0] = l*gain; pair[1] = r*gain;
}