public ConfigPayloadBuilder get(int index) {
            ConfigPayloadBuilder builder = elements.get(index);
            if (builder == null) {
                if (mode == ArrayMode.APPEND)
                    builder = append();
                else
                    builder = set(index);
            }
            return builder;
        }
Get payload builder in this array corresponding to index. If it does not exist, create a new one.

@param index of element to get.
@return The corresponding ConfigPayloadBuilder.
config/src/main/java/com/yahoo/vespa/config/ConfigPayloadBuilder.java