Altering a Built in Functiion

I can't get ImaginaryHuman's example to work. I've overloaded functions before, so I don't think I've got the syntax wrong, but

SuperStrict

Function DrawNothing(TextToDraw:String,XPos:Int,YPos:Int)
End Function

DrawText=DrawNothing

gives the error "Expression must be a variable". I'm using the latest SVN version, so if it works anywhere it should work for me.