clarified syntax: 'directories' and 'theories' belong together;

declare session directories;

tuned -- refining auto-update 15e9ed5b28fb;

isabelle update_cartouches -t;

ROOT cleanup: empty 'document_files' means there is no document;

clarified imports;

simplifie sessions;

support for 'chapter' specifications within session ROOT;

simplified session specifications: names are taken verbatim and current directory is default;

more explicit document = false to reduce warnings;
tuned;

more session ROOT files;