code: def _makemask(self, dest, res, pos):
          
          if (res is None or dest.bitwidth < res) and 0 < (dest.bitwidth - 64*pos) < 64:
              return '&0x{:X}'.format((1 << (dest.bitwidth % 64))-1)
          return ''
docs: Create a bitmask.
  
          The value being masked is of width `res`.
          Limb number `pos` of `dest` is being assigned to.