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 "../settings";
use_thy "termination";
use_thy "Induction";
use_thy "Nested1";
use_thy "Nested2";