Its the end of the episode that very nearly ruined the Weeping Angels forever (dont get me started. Or do, if you likeI love ranting). The Doctor and the audience have gotten some tantalizing clues about River Songs identity, pieces to add to the puzzle from Silence in the Library. River and the Doctor smile at each other, River dangles her foreknowledge smugly, and then she vanishes, laughing. And the Doctor gazes off into space. Amy asks him what hes thinking, and he says that Time can be re-written, and smiles. Amy doesnt know what hes talking about, but we do. Hes talking about The Library. River said then that if the Doctor ever truly surrendered to the implacable nature of deathand therefore of timeall the stars in all the worlds might just go dark. And here he is, thinking that time can be re-written. Maybe he can still save her. Or, maybe, hes thinking that there must be some sort of way to get out of that wedding.