I like to play the piano. Not very well, but I like to. The problem is finding sheet music on impulse. I hear a song, and would like to play it, but even if good sheet music exists for it, by the time I can get around to finding it somewhere, my interest may have wandered off to another song. Musicnotes has digital sheet music (buy and download a digital file), but their coverage isn't complete (what store's is?)
But there are tons of MIDI files out on the web. For example, Electric Light Orchestra MIDI files (today's song is Mr. Blue Sky). Some are good reproductions, some not so much. But they all are computer-readable lists of notes to play. What if I could convert a MIDI file into printed sheet music? There are MIDI disassemblers out there, but their output is hardly something you could play from.
Lilypond is a GNU music typesetter, and a very powerful one at that. It can turn music notation into a MIDI file, but what about the other way around? Has anyone heard of something like that?