Help debugging music function

[Vivian Barty-Taylor]

Could someone with knowhow help me to debug this music function? I want a 
function to create arrows in a Lilypond score:

arrow = #(define-music-function (parser location arg1 ) ( pair?)
  #{

\markup { \line \draw-line #arg1 \arrow-head #X #RIGHT ##t  }}

  #})

When I process:

\version "2.16.0"

\include "header.ly"


\score {

\new Staff {

c4_\arrow #'(10 . 0)

}
}

I get output:

Ontleden...
arrowtest.ly:10:3: fout: music function cannot return (#<procedure line-markup 
(layout props args)> ((#<procedure draw-line-markup (layout props dest)> (10 . 
0)) (#<procedure arrow-head-markup (layout props axis dir filled)> 0 1 #t)))
c4_
   \arrow #'(10 . 0)
Interpreting music...
programmeerfout: Not a music type

Thanks in advance,

Vivian.