Sheet music is copyrighted, and performing it requires paying a royalty. I believe that its performance can be debarred, but this is rare. If the performance is allowed, the musicians can interpret it however they wish, and they're not in violation of the copyright if the composer doesn't like their performance. The sheet music is much more than an API, because skilled musicians can "implement" it in real time. Even lead sheets (lead line with chord symbols) fall into that category. I was trying to find something that might be a legal precedent and thought a symphonic sketch might qualify. It can be copyrighted, although whether performing it requires paying a royalty might depend on its level of detail. Some sketches are much closer to finished sheet music than others. I believe that melodies in the sketch would stand up to copyright, but not general outlines. Perhaps the analogy to music isn't a very good one, in which case there might not be any informative precedents. Then I'd argue that an API which implies an underlying object model should be copyrightable, and that by default it would be subject to the same license as the underlying implementation.
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