<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def kill_tweens(self, obj = None):
 """Stop tweening an object, without completing the motion or firing the on_complete"""

        if obj is not None:
            try:
                del self.current_tweens[obj]
            except:
                pass
        else:
            self.current_tweens = collections.defaultdict(set)