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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 find_slot(cls, ctx):
 """ Finds an empty slot to set a hardware breakpoint. @see: clear_bp, set_bp @type ctx: dict( str S{->} int ) @param ctx: Thread context dictionary. @rtype: int @return: Slot (debug register) for hardware breakpoint. """

        Dr7  = ctx['Dr7']
        slot = 0
        for m in cls.enableMask:
            if (Dr7 & m) == 0:
                return slot
            slot += 1
        return None