San Francisco ? For some, it’s the red reindeer Christmas sweater. For others, it’s the diamond-encrusted dreidel. Whatever your worst gift nightmare might be, Amazon may soon give you a reason to cheer.

Amazon.com Inc., the source of many presents good, bad or ugly, has patented a system that would let people exchange unwanted gifts for those they actually want — even before they get them.

For now, though, it’s just an idea, so until Amazon figures out how to implement it, you may be stuck with that itchy sweater from Grandma, even if she bought it online.

Amazon did not return messages to speak about its plans.

Based on Amazon’s patent filing, recipients of unwanted gifts would be able to exchange items that are too big, too colorful or otherwise unwanted for something of equal value. They could also pay the difference for a more expensive item or get a gift certificate.

They could do this before they actually receive the gift, thus saving themselves the hassle of re-packaging and mailing the unwanted present.

If they wish, recipients could even send a thank you-note for the original gift. They can disclose to the sender that their gift has been converted, or simply keep it from them and just hope they don’t ask about it later.

If there’s a friend or family member with a history of giving unwanted presents — the filing calls her “Aunt Mildred” — users could select to convert all gifts from this person into something else.

Amazon says its idea would benefit gift-givers as well, as they can take a chance on a more interesting gift knowing that it can be returned.

Amazon, which is based in Seattle, had applied for the patent in March 2006. The company received it on Nov. 9.