docs: def convert(isbn, code='978'):
      """Convert ISBNs between ISBN-10 and ISBN-13.
  
      Note:
          No attempt to hyphenate converted ISBNs is made, because the
          specification requires that *any* hyphenation must be correct but
          allows ISBNs without hyphenation.
  
      Args:
          isbn (str): SBN, ISBN-10 or ISBN-13
          code (str): EAN Bookland code
  
      Returns:
          ``str``: Converted ISBN-10 or ISBN-13
  
      Raise:
          IsbnError: When ISBN-13 isn't convertible to an ISBN-10
  
      """
      isbn = _isbn_cleanse(isbn)
      if len(isbn) == 10:
          isbn = code + isbn[:-1]
          return isbn + calculate_checksum(isbn)
      else:
          if isbn.startswith('978'):
              return isbn[3:-1] + calculate_checksum(isbn[3:-1])
          else:
              raise IsbnError('Only ISBN-13s with 978 Bookland code can be '
                              'converted to ISBN-10.')