anchor.diff:  
           if (commands.containsKey(command)) {
             commands[command].add(new CommandEvent(event, command, args));
  +        } else {
  +          commandNotFound(new CommandEvent(event, command, args));
           }
         }
       });
  
positive.code:     }
      return true;
    }
  }
  
negative.code: 
    Map<String, StreamController<CommandEvent>> commands = {
    };
  
    Function commandNotFound = (CommandEvent event) => null;
  
    String prefix;
  
  