substitution for context

[Stefan Thomas]

Dear community,
I wanted to create a substitution-function with Scheme, where one could freely choose the context.
I don't know which kind of variable is necessesary for the context.
 Unfortunately I failed with:

\version "2.12.2"
%% hiermit kann man unterschiedliche TextSpanners auf Score- und Voice-Ebene haben!
Spantextleft = #(define-music-function (parser location CONTEXT left  )
                                    ( string? string? )
#{

\override $CONTEXT.TextSpanner #'bound-details #'left #'text = $left
#})

\relative c' {
  \Spantextleft #Voice c4 \startTextSpan d e f g1 \stopTextSpan 
}