[plt-scheme] Null syntax / Conditional definition

On Sat, 18 Jun 2005, Jordan Johnson might have said:

>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> 
> 
> On Saturday, June 18, 2005, at 09:39  AM, Eli Barzilay wrote:
> >  (define-syntax (OS-dep stx)
> >    (syntax-case ()
> >      [(_ u w)
> >       (case (system-type)
> >         [(unix)    #'u]
> >         [(windows) #'w])]))
> >  (OS-dep (define foo ...)
> >          (define foo ...))
> 
> Teeny change required in the second line:
> |(define-syntax (OS-dep stx)
> |    (syntax-case stx ()
> |      [(_ u w m)
> |       (case (system-type)
> |         [(unix)    #'u]
> |         [(windows) #'w]
> |         [(macosx) #'m])]))

What's the meaning of '_' in the expression after syntax-case?

Mike