class_docstr: Implements simple vocabulary.
  
  Parameters:
      special_tokens: tuple of tokens that shouldn't be counted.
      max_tokens: upper bound for number of tokens in the vocabulary.
      min_freq: minimal count of a token (except special tokens).
      pad_with_zeros: if True, then batch of elements will be padded with zeros up to length of
          the longest element in batch.
      unk_token: label assigned to unknown tokens.
      freq_drop_load: if True, then frequencies of tokens are set to min_freq on the model load.
      
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code: def keys(self):
      return (self[n] for n in range(self.len))
  
imports: from collections import Counter, defaultdict
  from itertools import chain
  from logging import getLogger
  from typing import Iterable, Optional, Tuple
  import numpy
  from deeppavlov.core.common.errors import ConfigError
  from deeppavlov.core.common.registry import register
  from deeppavlov.core.data.utils import zero_pad, is_str_batch, flatten_str_batch
  from deeppavlov.core.models.estimator import Estimator
label_desc: Use image node_id 9 for calling the SimpleVocabulary obj's underlying member method code with example usage: obj.keys() and returns: unknown