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.
code: def _cut_tail(self, key):
      with self._mutex:
          self._buckets[key].cut_tail()
  
imports: import collections
  import random
  import threading
label_desc: Use image node_id 14 for calling the Reservoir obj's underlying member method code with example usage: obj._cut_tail(key) without return types