'Remember the current set of objects as the comparison for all future calls to diff()
Called automatically on init, but can be called manually as well.'

def start(self):

    (refs, count, objs) = self.collect()
    for r in self.startRefs:
        self.forgetRef(self.startRefs[r])
    self.startRefs.clear()
    self.startRefs.update(refs)
    for r in refs:
        self.rememberRef(r)
    self.startCount.clear()
    self.startCount.update(count)
