public ConfigPayloadBuilder get(String key) {
            ConfigPayloadBuilder builder = elements.get(key);
            if (builder == null) {
                builder = put(key);
            }
            return builder;
        }
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