Crystal structure of ethyl 5′′-fluoro-2′′, 3-dioxo-6′, 7′, 8′, 8a'-tetra­hydro-2′H, 3H, 5′H-di­spiro­[benzo[b]thio­phene-2, 1′-indol­izine-3′, 3′′-indoline]-2′-carboxyl­ate. (March 2015)