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;

misc tuning;

tuned Change: eliminated redundant copy of document id;

eliminated redundant session documents database;
tuned;

use isabelle.Future;
tuned;

renamed current_document to recent_document (might be a bit older than current_change);
Change: explicit future value of Document.Change instead of implicit lookup in Session database;
Document_Model: apply Document.text_edits here (as future);