code: def source_start(base='', book_id='book'):
      
      repo_htm_path = "{book_id}-h/{book_id}-h.htm".format(book_id=book_id)
      possible_paths = ["book.asciidoc",
                        repo_htm_path,
                        "{}-0.txt".format(book_id),
                        "{}-8.txt".format(book_id),
                        "{}.txt".format(book_id),
                        "{}-pdf.pdf".format(book_id),
                       ]
  
      # return the first match
  
      for path in possible_paths:
          fullpath = os.path.join(base, path)
          if os.path.exists(fullpath):
              return path
  
      return None
docs: chooses a starting source file in the 'base' directory for id = book_id