Passing a parameter to Scheme

[Paul Hodges]

I am trying to make my first steps in actually modifying some Scheme.  I am using a procedure taken from the snippets repository to add a cross to a stem (e.g. to indicate sprechgesang):

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

speak = {

  \once \override Stem.stencil =

    #(lambda (grob)

       (let* ((x-parent (ly:grob-parent grob X))

              (is-rest? (ly:grob? (ly:grob-object x-parent 'rest))))

         (if is-rest?

             empty-stencil

             (ly:stencil-combine-at-edge

              (ly:stem::print grob)

              Y

              (- (ly:grob-property grob 'direction))

              (grob-interpret-markup grob

                                     (markup #:center-align #:fontsize -2

                                             #:musicglyph "noteheads.s2cross"))

              -2.3))))

}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

This works fine, writing "\speak c4" for instance.

However, I'd like to be able to parameterise the position of the cross which is hard-coded as -2.3, so that I can write "\speak #-3.8 c" for instance.  I've tried a couple of ways to write it, but I can't find one that works.  I suppose that I will need something like "lambda (grob posn)" to define the parameter for use at the end, but I can't see how to get the value of posn from the LilyPond source into the Scheme procedure.

Help?

Thanks,

Paul