code: def get_censor_char():
      
      global _censor_pool
      if not _censor_pool:
          # censor pool is empty. fill it back up.
          _censor_pool = list(_censor_chars)
      return _censor_pool.pop(random.randrange(len(_censor_pool)))
docs: Plucks a letter out of the censor_pool. If the censor_pool is empty,
      replenishes it. This is done to ensure all censor chars are used before
      grabbing more (avoids ugly duplicates).