public void removeChildren() {
        for (Children ch : childTrees) {
            ch.setInvalid();
            ch.setParent(null);
            ch.clearChildren();
        }
        childTrees.clear();
        resetCachedFromAndTo();
        ensureAtLeastOneSubTree();
    }
Removes all subtrees (both the subtrees themselves and their contents, which become invalidated).
Note that a new empty subtree is automatically added at index 0, since an AlternateSpanList always has at
least one subtree.
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java