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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 generate_custom(self, cpu, vcpu_num, fill_topology):
 """ Generate custom CPU model. This method attempts to convert the dict to XML, as defined by ``xmltodict.unparse`` method. Args: cpu(dict):
 CPU spec vcpu_num(int):
 number of virtual cpus fill_topology(bool):
 if topology is not defined in ``cpu`` and ``vcpu`` was not set, will add CPU topology to the generated CPU. Returns: lxml.etree.Element: CPU XML node Raises: :exc:`~LagoInitException`: when failed to convert dict to XML """


        try:
            cpu = utils.dict_to_xml({'cpu': cpu})
        except:
            # TO-DO: print an example here
            raise LagoInitException('conversion of \'cpu\' to XML failed')

        if not cpu.xpath('topology') and fill_topology:
            cpu.append(self.generate_topology(vcpu_num))
        return cpu