Which opera by Alban Berg was left unfinished at his death?

Lulu, left with its third act in short score when Berg died in 1935 of blood poisoning from an insect bite - at fifty, of something a course of antibiotics would now clear in a week.

It is built as a palindrome. The heroine rises through a series of men in the first half and falls through a corresponding series in the second, with the same singers doubling the roles - the doctor she ruins returns as a customer, and her final client is played by the same performer as Jack the Ripper. There is a silent film in the middle whose music runs forwards and then backwards, marking the exact centre of the mirror.

His widow blocked completion for forty years. Helene Berg refused permission, claiming her late husband's wishes communicated through a medium, and the opera was performed as two acts and a fragment until she died; Friedrich Cerha's completion was finally staged in Paris in 1979. Berg also embedded a fair amount of concealed autobiography in his scores, including his own initials and those of a married woman he was involved with, which was only decoded from an annotated score found long after his death.