@Override
    public int hashCode() {
        int result = super.hashCode();
        result = 31 * result + childTrees.hashCode();
        return result;
    }
Convenience method to add a span node to the child tree at index i. This is equivalent to calling
<code>
AlternateSpanList.children(i).add(node);
</code>

@param i    index
@param node span node
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java