one general list_all2_update_cong instead of two special ones

tuned comments;

standard Graph instances;

clarified signature -- avoid oddities of Iterable like Iterator.map; specific toString;

discontinued slightly odd Graph.del_nodes (inefficient due to full Table.map);

moved predicate relations and conversion rules between set and predicate relations from Predicate.thy to Relation.thy; moved Predicate.thy upwards in theory hierarchy

explicit is better than implicit

dropped dead code

prefer sorted Map/Set for canonical order of results -- pass ordering via fresh copy of empty; discontinued map_nodes, del_nodes conveniences -- avoid inefficient mapValues wrapper (this is not Table.map from ML); tuned signature;

tuned imports;