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