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.ML";
use_thy "Even";
use_thy "Mutual";
use_thy "Star";
use_thy "AB";
use_thy "Advanced";