Scheme attempt gives 'unknown escape string' error - any hints appreciated

[Morten Jagd Christensen]

Hi all

Im writing a small sceme function to be able to create examples of guitar fingerings
for various scales.

basically i create a music function "fretsc" to be called for example as

\fretsc "(6 1) (6 3) (5 0)"    to indicate the position and sequence of notes in a scale.

However I get an "unknown escape string" for \fret while compiling.

Anyone have an idea what Im doing wrong?

Here is the example code if you comment out the last line you can see the kind of fret diagram I am
working on.

\version "2.14.0"

fretsc = #(define-music-function (parser location s1 )
                     (string?)
#{
\markup{\override #'(fret-diagram-details . (
                   (finger-code . in-dot)
                   (number-type . arabic)
                   (label-dir . -1)
                   (orientation . landscape)
                   (dot-radius  . 0.5)
                   (fret-count . 5)
                   (top-fret-thickness . 5)
                   (barre-type . straight)
                   (xo-padding . 0.3) )) {
        \fret-diagram-verbose #'(
                                   $s1
                                 )
      }
    }
#})

ddd =  \markup{\override #'(fret-diagram-details . (
                   (finger-code . in-dot)
                   (number-type . arabic)
                   (label-dir . -1)
                   (orientation . landscape)
                   (dot-radius  . 0.5)
                   (fret-count . 5)
                   (top-fret-thickness . 5)
                   (barre-type . straight)
                   (xo-padding . 0.3) )) {
        \fret-diagram-verbose #'(
                                 (place-fret 5 3 1)
                                 (place-fret 4 5 2)
                                 (place-fret 3 5 3)
                                 (place-fret 2 5 4)
                                 (place-fret 1 3 5)
                                 (place-fret 1 4 6)
                                 )
      }
    }

\score {
   \new Staff {\relative c' {  a'^\ddd b c d  }}
}
\fretsc "(6 3 1) (5 3 2)"



... and here are the errors:
Processing `/Users/mjc/Desktop/all music/noder/snippets/fret.ly'
Parsing...
<string>:2:0: error: syntax error, unexpected \markup

\markup{\override #(quote (fret-diagram-details (finger-code . in-dot) (number-type . arabic) (label-dir . -1) (orientation . landscape) (dot-radius . 0.5) (fret-count . 5) (top-fret-thickness . 5) (barre-type . straight) (xo-padding . 0.3))) {
<string>:3:8: error: unknown escaped string: `\fret'

        \fret-diagram-verbose #(quote (lilyvartmpbg))
/Users/mjc/Desktop/all music/noder/snippets/fret.ly:47:0: error: errors found, ignoring music _expression_

\fretsc "(6 3 1) (5 3 2)"
success: Compilation successfully completed