gives me <string>:3:4: error: syntax error, unexpected STRING
{ a4 b c d }
Now, my Scheme programming is very rusty, so I'm probably doing something silly.
Also, I'm using a string because that appeared simpler. I would really want to use ly:music as the parameter type, but then I need some code to expand the music list, and I don't have a clue as to what that would be.