@Override
    public AlternateSpanList add(SpanNode node) {
        return add(0, node);
    }
Adds a child node to the <strong>first</strong> subtree of this AlternateSpanList. Note
that it might be a good idea to call {@link #sortSubTreesByProbability()} first.

@param node the node to add.
@return this, for call chaining
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java