docs: public String getSingleLineUsage() {
          StringBuilder writer = new StringBuilder();
          writer.append(program);
  
          // first just list up all the unary short opts.
          StringBuilder sh = new StringBuilder();
          // Only include the first short name form the flag.
          options.stream()
                 .filter(opt -> opt instanceof Flag)
                 .filter(opt -> opt.getShortNames().length() > 0)
                 .forEachOrdered(opt -> sh.append(opt.getShortNames().charAt(0)));
  
          if (sh.length() > 0) {
              writer.append(" [-").append(sh.toString()).append(']');
          }
  
          for (BaseOption opt : options) {
              if (opt instanceof Flag && opt.getShortNames().length() > 0) {
                  // already included as short opt.
                  continue;
              }
              String usage = opt.getSingleLineUsage();
              if (usage != null) {
                  writer.append(' ').append(usage);
              }
          }
          for (BaseArgument arg : arguments) {
              String usage = arg.getSingleLineUsage();
              if (usage != null) {
                  writer.append(' ').append(usage);
              }
          }
  
          return writer.toString();
      }