class_docstr: An nbconvert preprocessor that substitutes one set of HTML data
  output for another, adding annotation to the output as required.
  
  The constructor accepts the notebook format version and a
  substitutions dictionary:
  
  {source_html:(target_html, annotation)}
  
  Where the annotation may be None (i.e. no annotation).
code: def __call__(
      self, nb, resources
  ):  # Temporary hack around 'enabled' flag
      return self.preprocess(nb, resources)
  
imports: import ast
  from nbconvert.preprocessors import Preprocessor
label_desc: Use image node_id 2 for calling the Substitute obj's underlying member method code with example usage: obj.__call__(nb, resources) and returns: self