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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 checksum(*paths, **kwargs):
    '''
    Return if the signature of a RPM file is valid.

    root
        use root as top level directory (default: "/")

    CLI Example:

    .. code-block:: bash

        salt '*' lowpkg.checksum /path/to/package1.rpm
        salt '*' lowpkg.checksum /path/to/package1.rpm /path/to/package2.rpm
    '''
    ret = dict()

    if not paths:
        raise CommandExecutionError("No package files has been specified.")

    cmd = ['rpm']
    if kwargs.get('root'):
        cmd.extend(['--root', kwargs['root']])
    cmd.extend(['-K', '--quiet'])
    for package_file in paths:
        cmd_ = cmd + [package_file]
        ret[package_file] = (bool(__salt__['file.file_exists'](package_file)) and
                            not __salt__['cmd.retcode'](cmd_,
                                                        ignore_retcode=True,
                                                        output_loglevel='trace',
                                                        python_shell=False))

    return ret