How to modify topic's marker

[anstarword]

I only find addMarker in topic.py, but how to modify a topic's marker?
For example, i want to replace all the topices' flag to priority.
i want to

1. get all the topic tree
2. find which topic has the marker of flag
3. delete the marker of flag
4. add a marker of priority to topic

But i don't find how to delete a marker from topic

In the other hand, when i use recursion to add topic.addMarker, i get the decode error. I don't know why?

File "C:/Users/ayan/IdeaProjects/XmindReader/XmindReader-GenerateReadme2.py", line 57, in findTestCase
topic.addMarker(MarkerId(newMarkerID))
File "C:\Python34\lib\site-packages\xmind-0.1a0-py3.4.egg\xmind\core\topic.py", line 133, in addMarker
File "C:\Python34\lib\site-packages\xmind-0.1a0-py3.4.egg\xmind\core\markerref.py", line 131, in init
File "C:\Python34\lib\site-packages\xmind-0.1a0-py3.4.egg\xmind\core\mixin.py", line 25, in init
File "C:\Python34\lib\site-packages\xmind-0.1a0-py3.4.egg\xmind\core__init__.py", line 204, in init
AttributeError: 'str' object has no attribute 'decode'