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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_rfu():
 """ Returns a list of al "regular file urls" for all plugins. """

    global _rfu
    if _rfu:
        return _rfu

    plugins = plugins_base_get()
    rfu = []
    for plugin in plugins:
        if isinstance(plugin.regular_file_url, str):
            rfu.append(plugin.regular_file_url)
        else:
            rfu += plugin.regular_file_url

    _rfu = rfu

    return rfu