Slurs in scheme?

[Joe Neeman]

Sorry if I've missed the documentation on this, but I'm trying to build 
music expressions involving slurs in scheme. This is the first time I've 
tried to do anything useful in scheme, so please be patient :)

As a simple example, here I'm trying to write a function that expands 
"a" into something like "a( a)":

slur =
#(def-music-function
 (parser location m)
 (ly:music?)

 (make-sequential-music
   (list
     m
     (make-span-event 'SlurEvent START)
     m
     (make-span-event 'SlurEvent STOP))))

{
   b \slur a
}

But I don't get the slur in the output. I've also tried similar things 
with ties and I've tried changing the order of the elements in the list.

Can someone give me a pointer here? (version 2.6, by the way)

Thanks,
Joe