msc {
   "User", "Quit GUI", "Session Manager", "Application";

   "User" -> "Quit GUI" [label = "I'm leaving"];
   "Quit GUI" -> "Session Manager" [label = "I'm thinking about quitting"];
   --- [label = "For each application"];
   "Session Manager" -> "Application" [label = "SaveYourself"];
   --- [label = "If the application wants to interact"];
   "Session Manager" <- "Application" [label = "InteractRequest"];
   --- [label = "If the application does not want to interact"];
   ... [label = "A bunch of work"];
   "Session Manager" <- "Application" [label = "SaveYourselfDone"];
   --- [label = "Everyone Now"];
   "Session Manager" -> "Application" [label = "ShutdownCancelled"];
   "Quit GUI" <- "Session Manager" [label = "List of apps that asked for interaction"];
   --- [label = "If the list is non-zero"];
   "User" <- "Quit GUI" [label = "How would you like to handle these?"];
   --- [label = "else"];
   "Quit GUI" -> "Session Manager" [label = "Okay, really shutdown"];
}