Gravar-mail: rac-5′′-(4-Fluoro­benzyl­idene)-1′-(4-fluoro­phen­yl)-1′′-methyl-1′,2′,3′,5′,6′,7′,8′,8a′-octa­hydro­dispiro­[acenaphthyl­ene-1,3′-indolizine-2′,3′′-piperidine]-2,4′′(1H)-dione