presto-main/src/main/java/io/prestosql/operator/unnest/Unnester.java/Unnester/processCurrentAndAdvance
class Unnester:    public final void processCurrentAndAdvance(int requiredOutputCount)
    {
        checkState(currentPosition >= 0 && currentPosition < getInputEntryCount(), "position out of bounds");
        processCurrentPosition(requiredOutputCount);
        currentPosition++;
    }