docs: private void setupEmitters() {
      // set the iterators
      for (int i = 0; i < dps.length; i++) {
        iterators[i] = dps[i].iterator();
        if (!iterators[i].hasNext()) {
          current_values[i] = null;
          emitter_values[i] = null;
        } else {
          current_values[i] = iterators[i].next();
          emitter_values[i] = new ExpressionDataPoint(dps[i]);
          emitter_values[i].setIndex(i);
        }
      }
    }