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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 extract_native_client_tarball(dir):
    r'''
    Download a native_client.tar.xz file from TaskCluster and extract it to dir.
    '''
    assert_valid_dir(dir)

    target_tarball = os.path.join(dir, 'native_client.tar.xz')
    if os.path.isfile(target_tarball) and os.stat(target_tarball).st_size == 0:
        return

    subprocess.check_call(['pixz', '-d', 'native_client.tar.xz'], cwd=dir)
    subprocess.check_call(['tar', 'xf', 'native_client.tar'], cwd=dir)
    os.unlink(os.path.join(dir, 'native_client.tar'))
    open(target_tarball, 'w').close()