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