generate questions: In September 1998, the Fraunhofer Institute sent a letter to several developers of MP3 software stating that a license was required to "distribute and/or sell decoders and/or encoders". The letter claimed that unlicensed products "infringe the patent rights of Fraunhofer and Thomson. To make, sell and/or distribute products using the [MPEG Layer-3] standard and thus our patents, you need to obtain a license under these patents from us."
When did the Fraunhofer institute send out a letter? {sep_token} What did the letter claim would be required to distribute and/or sell decoders and/or encoders? {sep_token} What does selling unlicensed products mean that the seller is doing? {sep_token} What can products not use if they want to avoid infringing on patent rights?