class_docstr: TextStats contains following basic information:
code: def document_frequency(self):
      return self._document_frequency
  
imports: from typing import Dict, Optional, Tuple
  from xai.data.abstract_stats import AbstractStats
  from xai.data.constants import STATSKEY
  from xai.data.exceptions import InvalidTypeError, InvalidSizeError
label_desc: Use image node_id 12 for calling the TextStats obj's underlying member method code with example usage: obj.document_frequency() and returns: self