public void sortSubTreesByProbability() {
        resetCachedFromAndTo();
        Collections.sort(childTrees, childComparator);
    }
Sorts the subtrees under this AlternateSpanList by descending probability, such that the most probable
subtree becomes the first subtree, and so on.
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java