1,2,3,5a,6,11,12,14a‐Octa­hydro‐9‐meth­oxy‐12‐(2‐methyl­prop‐1‐en­yl)‐5H,14H‐pyrrolo­[1′′,2′′:4′,5′]pyrazino[1′,2′:1,6]pyrido[3,4‐b]indole‐5,14‐dione