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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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 ''