private Collection<Annotation> getAnnotations() {
        return annotations.annotations();
    }
Ensures consistency of the tree in case SpanNodes have been removed, and there are still
Annotations pointing to them. This method has a maximum upper bound of O(3nm), where n is the
total number of Annotations, and m is the number of SpanNodes that had been removed from the tree.
The lower bound is Omega(n), if no SpanNodes had been removed from the tree.
document/src/main/java/com/yahoo/document/annotation/SpanTree.java