code: private String getNextListIndexSuffix(Map<String, String> options, String key) {
          if (!options.containsKey(key)) {
              return "";
          } else {
              int i = 1;
              while (options.containsKey(key + "." + i)) {
                  i++;
              }
              return "." + i;
          }
      }
docs: check for the next key with a suffix like ".1" which is not already set