minor symmetry breaking for codatatypes like llist

fix bug with mutually recursive and nested codatatypes

fix minor bug in sym breaking

modernized specifications; tuned headers;

Document_Model: include token marker here;

tuned;

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;