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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 update_boot_system_use(self, boot_sys_use):
        # type: (bytes) -> None
        '''
        A method to update the boot system use field of this Boot Record.

        Parameters:
         boot_sys_use - The new boot system use field for this Boot Record.
        Returns:
         Nothing.
        '''
        if not self._initialized:
            raise pycdlibexception.PyCdlibInternalError('Boot Record not yet initialized')

        self.boot_system_use = boot_sys_use.ljust(197, b'\x00')