In September 1998, the Fraunhofer Institute sent a letter to several developers of MP3 software stating that <answer>a license</answer> 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 <answer>a license</answer> under these patents from us."
What did the letter claim would be required to distribute and/or sell decoders and/or encoders?
a license