There are a couple of file formats that only have one library supporting them: hocon (YAMLConfig, as seen in day two), .ini (Configuration), and .properties (Configuration). Bonus fact: if you check the index right now, you would see that the json5 ConfigFormat only has one usage. this is because when i made the first 8 or so entries to the index, i didn't know there was a difference in between json and json5, so only libraries added post-java-port are labeled as json5 correctly. and by chance, only one library added post-java-port supports json5.