class_docstr: A map-to-arrays dict, with deterministic Reservoir Sampling.
  
  Store each reservoir bucket by key, and each bucket is a list sampling
  with reservoir algorithm.
docstr: Pop the last item in reservoir buckets.
  
  Sometimes the tail of the retrieved data is abnormal 0. This
  method is used to handle this problem.
  
  Args:
      run: Identity of one tablet.
      tag: Identity of one record in tablet.
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code: def cut_tail(self, run, tag):
      """Pop the last item in reservoir buckets.
  
      Sometimes the tail of the retrieved data is abnormal 0. This
      method is used to handle this problem.
  
      Args:
          run: Identity of one tablet.
          tag: Identity of one record in tablet.
      """
      key = run + "/" + tag
      self._cut_tail(key)
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 15 for calling the Reservoir obj's underlying member method code with example usage: obj.cut_tail(run, tag) without return types