public int numChildren(int i) {
        return children(i).size();
    }
Recursively traverses all children (not only leaf nodes) of the given subtree of this AlternateSpanList, in a
depth-first fashion.
The ListIterator only supports iteration forwards, and the optional operations that are implemented are
remove() and set(). add() is not supported.

@param i the index of the subtree to iterate over
@return a ListIterator which recursively traverses all children and their children etc. of the given subtree of this AlternateSpanList.
@see java.util.ListIterator
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java