public void clearChildren(int i) {
        Children c = childTrees.get(i);
        if (c != null) {
            c.clearChildren();
        }
    }
Clears a given subtree (the subtree itself is kept, but its contents are cleared and become invalidated).

@param i the index of the subtree to clear
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java