protected List<SpanNode> children(int i) {
        return childTrees.get(i).children();
    }
Returns a modifiable {@link List} of child nodes of the specified subtree.

@param i the index of the subtree to search
@return a modifiable {@link List} of child nodes of the specified subtree
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java