(doc help) escaping a ly:music? variable

[Graham Percival]

Quote from upcoming docs; please fill in the #### and fix the lilypond 
code.  :)

Cheers,
- Graham

-----
Even music expressions can be passed in.  Note that since we
want an articulation attached to the second variable, we
must #####.

pattern = #(define-music-function (parser location x y) (ly:music? 
ly:music?)
#{
  $x e8-. a-. b-. $y-.-> b-. a-. e-.
#})

\relative c''{
  \pattern c8 c8
  \pattern d8 ais8
  \pattern cis8 des8
}
----