/** Called the first time next() or skipTo() is called to
* initialize <code>scorerDocQueue</code>.
*/
private void initScorerDocQueue() throws IOException {
Iterator si = subScorers.iterator();
scorerDocQueue = new ScorerDocQueue(nrScorers);
while (si.hasNext()) {
Scorer se = (Scorer) si.next();
if (se.nextDoc() != NO_MORE_DOCS) { // doc() method will be used in scorerDocQueue.
scorerDocQueue.insert(se);
}