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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 plugin_get_rfu(plugin):
 """ Returns "regular file urls" for a particular plugin. @param plugin: plugin class. """

    if isinstance(plugin.regular_file_url, str):
        rfu = [plugin.regular_file_url]
    else:
        rfu = plugin.regular_file_url

    return rfu