chosen example can be unchosen via ctrl also

src/message/show_messages.py

@@ -49,6 +49,10 @@ class ShowExampleEdit(CtrlClickMessage):
         
     def update_model_ctrl(self, model):
         example = self.get_arg(0, Example)
+        
+        if example in model.chosen_examples:
+            model.chosen_examples.remove(example)
+        else:
             model.chosen_examples.append(example)
         
     def reset(self):