code: def preprocess_notebook(ntbk, cr):
      
  
      # Iterate over cells in notebook
      for n in range(len(ntbk['cells'])):
          # Only process cells of type 'markdown'
          if ntbk['cells'][n]['cell_type'] == 'markdown':
              # Get text of markdown cell
              txt = ntbk['cells'][n]['source']
              # Replace links to online docs with sphinx cross-references
              txt = cr.substitute_url_with_ref(txt)
              # Replace current cell text with processed text
              ntbk['cells'][n]['source'] = txt
docs: Process notebook object `ntbk` in preparation for conversion to an
      rst document.  This processing replaces links to online docs with
      corresponding sphinx cross-references within the local docs.
      Parameter `cr` is a CrossReferenceLookup object.