def _makemask(self, dest, res, pos):
        """Create a bitmask.

        The value being masked is of width `res`.
        Limb number `pos` of `dest` is being assigned to.
        """
        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 ''
Create a bitmask.

        The value being masked is of width `res`.
        Limb number `pos` of `dest` is being assigned to.