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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 get_available_themes():
 """ Iterator on available themes """

    for d in settings.TEMPLATE_DIRS:
        for _d in os.listdir(d):
            if os.path.isdir(os.path.join(d, _d)) and is_theme_dir(_d):
                yield _d