misc tuning -- produce reverse_edits at most once (note that foldRight produces a reversed list internally, while recursion is infisible due to small stack vs. large stack frames on JVM);

editor mode;

Text_Edit.convert/revert;

renamed to_current to convert, and from_current to revert;

Change.Snapshot: include from_current/to_current here, with precomputed changes;

explicit Change.Snapshot and Document.Node; misc tuning and clarification; Document_View: explicitly highlight outdated command status;