docs: protected String processOption(DefaultCommandLine cl, String arg) {
          if (arg.length() < 2) {
              return null;
          }
  
          if (arg.charAt(1) == 'D' && arg.contains("=")) {
              processSystemArg(cl, arg);
              return null;
          }
  
          arg = (arg.charAt(1) == '-' ? arg.substring(2, arg.length()) : arg.substring(1, arg.length())).trim();
  
          if (arg.contains("=")) {
              String[] split = arg.split("=");
              String name = split[0].trim();
              validateOptionName(name);
              String value = split[1].trim();
              if (declaredOptions.containsKey(name)) {
                  cl.addDeclaredOption(declaredOptions.get(name), value);
              } else {
                  cl.addUndeclaredOption(name, value);
              }
              return null;
          }
  
          validateOptionName(arg);
          if (declaredOptions.containsKey(arg)) {
              cl.addDeclaredOption(declaredOptions.get(arg));
          } else {
              cl.addUndeclaredOption(arg);
          }
          return arg;
      }