@Override
    public SpanTree getSpanTree() {
        if (parent == null) {
            return null;
        }
        return parent.getSpanTree();
    }
Returns the SpanTree that this node belongs to, if any.

@return the SpanTree that this node belongs to, or null if it is not yet added to a SpanTree.
document/src/main/java/com/yahoo/document/annotation/SpanNode.java