removed slightly odd Isar_Document.init;
simplified begin_document: associate empty command/state with no_id, which is implicit start;
replaced make_id by create_id (cf. Scala version);
eliminated CRITICAL/Unsynchronized.ref in favour of Synchronized.var;
tuned;
use_thy "FP0";
use_thy "FP1";
use_thy "RECDEF";
use_thy "Rules";
use_thy "Sets";
use_thy "Ind";