code: private static List<Integer> getUsedIndexes() {
          File folder = getMotdFileOutputStreamProvider().getMOTDFolder();
          List<Integer> res = new ArrayList<>();
  
          for (File file : folder.listFiles()) {
              if (file.isFile() && file.getName().matches(getRegexToMatchSerializedMOTs())) {
                  res.add(getIndexFromFilename(file.getName()));
              }
          }
  
          return res;
      }
docs: Returns a list of indexes already used by serialized {@code MOTD}s
  
  @return A list of indexes already used by serialized {@code MOTD}s