public MapBuilder getMap(String name) {
        MapBuilder a = mapBuilderMap.get(name);
        if (a == null) {
            validateMap(name);
            a = new MapBuilder(configDefinition, name);
            mapBuilderMap.put(name, a);
        }
        return a;
    }
Create a new map where new values may be added.

@param name Name of map.
@return Map builder supporting adding elements to it.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadBuilder.java