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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def FilterItems(self, filterFn):
 """Filter items in a ReservoirBucket, using a filtering function. Filtering items from the reservoir bucket must update the internal state variable self._num_items_seen, which is used for determining the rate of replacement in reservoir sampling. Ideally, self._num_items_seen would contain the exact number of items that have ever seen by the ReservoirBucket and satisfy filterFn. However, the ReservoirBucket does not have access to all items seen -- it only has access to the subset of items that have survived sampling (self.items). Therefore, we estimate self._num_items_seen by scaling it by the same ratio as the ratio of items not removed from self.items. Args: filterFn: A function that returns True for items to be kept. Returns: The number of items removed from the bucket. """

    with self._mutex:
      size_before = len(self.items)
      self.items = list(filter(filterFn, self.items))
      size_diff = size_before - len(self.items)

      # Estimate a correction the number of items seen
      prop_remaining = len(self.items) / float(
          size_before) if size_before > 0 else 0
      self._num_items_seen = int(round(self._num_items_seen * prop_remaining))
      return size_diff