public ModelGraphBuilder addBuilder(ConfigModelBuilder<? extends ConfigModel> builder) {
        builders.add(builder);
        return this;
    }
Add a {@link com.yahoo.config.model.builder.xml.ConfigModelBuilder} to this graph.

@param builder The {@link com.yahoo.config.model.builder.xml.ConfigModelBuilder} to add.
@return this for convenience
config-model/src/main/java/com/yahoo/config/model/graph/ModelGraphBuilder.java