code: def to_serializable(self):
      contents = super(SequenceVocabulary, self).to_serializable()
  
      contents.update(
          {
              "mask_token": self._mask_token,
              "begin_seq_token": self._begin_seq_token,
              "end_seq_token": self._end_seq_token,
          }
      )
      del contents["add_unk"]
      return contents
  
imports: from typing import Dict
label_desc: Use image node_id 2 for calling the SequenceVocabulary obj's underlying member method code with example usage: obj.to_serializable() and returns: contents