substitution for numerator

[Stefan Thomas]

Dear community,
I've tried to make a substitution for the numerator in scaled durations, but without sucess.
I've tried it with:

numerator = #(define-music-function (parser location x) (number?)
  #{ % in der folgenden Zeile Lilypondcode eingeben 
    c1*$x/3
#})
themusic = {
  \numerator #4
}

I assume, that number? is't the right type of variable, am I right?