Cannot set MIDI tempo Programmatically

[musicologist1964]

Hi,

I'm having a problem setting the MIDI tempo from within a function.  When I
compile the file below, I get the error message:

GNU LilyPond 2.11.35
Processing `test.ly'
Parsing...
<string>:3:11: error: syntax error, unexpected SCM_TOKEN, expecting DIGIT or
UNSIGNED or DURATION_IDENTIFIER
    \tempo 
           #lilyvartmpb = \lilyvartmpc
test.ly:26:0: error: errors found, ignoring music expression

\relative c'

It seems that lilypond is complaining that the parameter Duration is not a
number.  I've tried various ways of getting around this, but it doesn't seem
to work.  If I comment out the \tempo line, I get a music file (
http://www.nabble.com/file/p14336623/test.png test.png ) with the
appropriate tempo marking, but it has no effect on the MIDI tempo.

Any ideas?

Cheers,

Garry.

-------------------------------------------------------------------------------------
\version "2.11.35"
\include "english.ly"

GRtempoMark = 
#(define-music-function (parser location Padding Text    Duration PerMinute)
                                       ( number? string? string?  string?)
  #{
    \once \override Score.MetronomeMark #'transparent = ##t
    \tempo #$Duration = $PerMinute

    \once \override Score.RehearsalMark #'self-alignment-X = #left 
    \once \override Score.RehearsalMark #'padding = #$Padding 
    \mark \markup
    {
      \bold $Text
      (
      \smaller \general-align #Y #DOWN \note #$Duration #1
      =
      \smaller \general-align #Y #DOWN \italic "ca."
      \general-align #Y #DOWN $PerMinute
      )
    }
  #}
)
  
\relative c'
{
  \GRtempoMark #2 #"Andante" #"4" #"60"
  c4 
}
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