def __init__(self):
        self.startRefs = {}        ## list of objects that exist when the tracker is initialized {oid: weakref}
                                   ##   (If it is not possible to weakref the object, then the value is None)
        self.startCount = {}       
        self.newRefs = {}          ## list of objects that have been created since initialization
        self.persistentRefs = {}   ## list of objects considered 'persistent' when the last diff() was called
        self.objTypes = {}