tuned signature of isabelle.Position;

maintain generic session entities -- cover commands, states, etc. (but not yet documents);

unified Command and Command_State, eliminated separate Accumulator;
Command.finish_static: idempotent transition from static to dynamic mode, taking snapshot of final state (which will never change later);

removed experimental history panel;

more explicit modeling of Command and Command_State as Session.Entity;
misc tuning;

updated;

tuned;

tuned caret_listener/selected_command;

misc tuning;

eliminated global Session.document_0 -- did not work due to hardwired id;
more precise Session.begin_document, avoid race on var prover;
replaced slightly odd Session.document_versions by Sassion.documents table (cf. src/Pure/Isar/isar_document.ML);
simplified edit_document in ML: initial empty command is identified by "";
misc tuning;