code: private void validateArgumentDefinitions() {
          for (final NamedArgumentDefinition mutexSourceDef : namedArgumentDefinitions) {
              for (final String mutexTarget : mutexSourceDef.getMutexTargetList()) {
                  final NamedArgumentDefinition mutexTargetDef = namedArgumentsDefinitionsByAlias.get(mutexTarget);
                  if (mutexTargetDef == null) {
                      throw new CommandLineException.CommandLineParserInternalException(
                              String.format("Argument '%s' references a nonexistent mutex argument '%s'",
                                      mutexSourceDef.getArgumentAliasDisplayString(),
                                      mutexTarget)
                      );
                  } else {
                      // Add at least one alias for the mutex source to the mutex target to ensure the
                      // relationship is symmetric for purposes of usage/help doc display.
                      mutexTargetDef.addMutexTarget(mutexSourceDef.getLongName());
                  }
              }
          }
      }
docs: arguments involved.