@Override
    void setInvalid() {
        //invalidate ourselves:
        super.setInvalid();
        //invalidate all child trees
        for (Children ch : childTrees) {
            ch.setInvalid();
        }
    }
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