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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 register_plugins(self):
 """Load plugins listed in config variable 'PLUGINS'."""

        registered = set()
        for plugin_fqdn in chain(self.APP_PLUGINS, self.config["PLUGINS"]):
            if plugin_fqdn not in registered:
                self.register_plugin(plugin_fqdn)
                registered.add(plugin_fqdn)