explicit Document.State value, instead of individual state variables in Session, Command, Document;
Session.snapshot: pure value based on Document.State;
Document.edit_texts: no treatment of state assignment here;

edit_document: more precise status position;

added get_after convenience;

more basic notion of unparsed input;

simplified/clarified Change: transition prev --edits--> result, based on futures;
Session.history: plain List instead of somewhat indirect Change.ancestors;
tuned;

moved snapshot to Session (cf. 96b22dfeb56a);

Change: eliminated id, which is merely the resulting document id and is only required in joined state anyway;
Document.edit_text: create new document id here;

clarified "state" (accumulated data) vs. "exec" (execution that produces data);
generic type Document.id (ML) / Document.ID;

misc tuning and simplification;

specific command state;