simplified/refined document model: collection of named nodes, without proper dependencies yet;
moved basic type definitions for ids and edits from Isar_Document to Document;
removed begin_document/end_document for now -- nodes emerge via edits;
edits refer to named nodes explicitly;
use_thy "FP0";
use_thy "FP1";
use_thy "RECDEF";
use_thy "Rules";
use_thy "Sets";
use_thy "Ind";