public void sortChildren(int i) {
        Children children = childTrees.get(i);
        Collections.sort(children.children());
    }
Sorts children in subtree i by occurrence in the text covered.

@param i the index of the subtree to sort
@see SpanNode#compareTo(SpanNode)
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java