The Angsuman's Feed Copyrighter Plugin adds a copyright message to your feed items. No customization is required. This acts as a deterrant to feed-stealers who unscrupulously and blatantly displays your full feed contents (often with their own advertisements) in their site(s) without your permission.
It doesn't affect when only excerpts are shown nor it adds anything to the feeds when you decide to publish only article summary in your feeds.
This is of the class of upload-activate-and-forget Plugins, Plugins that simply work.
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