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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 list_extensions():
    '''
    List up available extensions.

    Note:
        It may not work on some platforms/environments since it depends
        on the directory structure of the namespace packages.

    Returns: list of str
        Names of available extensions.

    '''
    import nnabla_ext.cpu
    from os.path import dirname, join, realpath
    from os import listdir
    ext_dir = realpath((join(dirname(nnabla_ext.cpu.__file__), '..')))
    return listdir(ext_dir)