docs: def i2osp(self, long_integer, block_size):
          'Convert a long integer into an octet string.'
          hex_string = '%X' % long_integer
          if len(hex_string) > 2 * block_size:
              raise ValueError('integer %i too large to encode in %i octets' % (long_integer, block_size))
          return a2b_hex(hex_string.zfill(2 * block_size))