code: public static void checkOptional(OptionSet options, List<String> opts)
              throws VoldemortException {
          List<String> optCopy = Lists.newArrayList();
          for(String opt: opts) {
              if(options.has(opt)) {
                  optCopy.add(opt);
              }
          }
          if(optCopy.size() > 1) {
              System.err.println("Conflicting options:");
              for(String opt: optCopy) {
                  System.err.println("--" + opt);
              }
              throw new VoldemortException("Conflicting options detected.");
          }
      }
docs: Checks if there's at most one option that exists among all opts.
  
  @param parser OptionParser to checked
  @param opts List of options to be checked
  @throws VoldemortException