Conditional ambitus

[Kevin Stolt]

Hi,
I'm trying to conditionally add ambitus based on a variable definition.  I'm 
having trouble figure out how to do it.

\version "2.16.2"
isAmbitus = #f
includeAmbitus = #(define-music-function
  (parser location)
  ()
  (if isAmbitus
  #{ \with { \consists "Ambitus_engraver" } #}
  #{ #} ))

%When I try to use includeAmbitus:
\score {
\new Voice = "one" \includeAmbitus {e' f' g'}
}



It gives an error about an unexpected EVENT_IDENTIFIER
Is this there a different function type I should be using
instead of define-music-function?  
Thank you.
Kevin