clarified modules;

more reactive interrupts (via Future.cancel);

Document.Snapshot: support for multiple snippet_commands; clarified Command.rich_text: prefer explicit id, e.g. from message serial; clarified Pretty_Text_Area.update: Protocol_Message.provide_serial; clarified Pretty_Text_Area.format_rich_texts, with separate formatting of messages;

more robust: make double-sure that this is the correct output, not a different version from concurrent GUI_Thread.later;