anchor.diff:      if(commandInvocation == null) {
         // TODO: Move this to an earlier UsageException instead ?
         if(usage != null && usage.commands.isNotEmpty) {
  -        print('Must specify a sub-command.\n');
  -        _printHelp(usage);
  +        _handleUsageError(usage, 'Must specify a sub-command.');
         }
         return;
       }
  
positive.code:   /// Handles successfully validated [commandInvocation].
    _handleResults(CommandInvocation commandInvocation);
  
    /// Prints help information for the associated command or sub-command thereof
    /// at [commandPath].
    // TODO: Integrate with Loggers.
    _printHelp(Usage helpUsage) {
      print(getUsageFormatter(helpUsage).format());
    }
  
    bool _checkHelp(CommandInvocation commandInvocation) {
  
negative.code:       // TODO: ArgParser.parse throws FormatException which does not indicate
        // which sub-command was trying to be executed.
        var helpUsage = e is UsageException ? e.usage : usage;
        _handleUsageError(helpUsage, e);
        return;
      }
  
  