back to explicit management of documents -- not as generic Session.Entity -- to avoid ill-defined referencing of new states; recent_document: require finished state assignment; explicitly typed Session.lookup_command;

slightly more uniform/robust handling of visible document; uniform changed_delay for view updates; misc tuning;

recovered explicit Changed.id (copy of future document) -- avoids premature joining of result; Change.ancestors: frugal iterator;

more explicit treatment of command/document state; misc tuning and clarification;

removed unused document_change bus;

tuned;

tuned;