include groups from 'chapter_definition' in session info, based on the state of chapter_defs after processing all ROOT files (thus the declaration order does not matter);

proper chapter_definition to prevent odd definitions by users;

tuned;

tuned signature;

proper order;

tuned whitespace for presentation;

clarified signature of Sessions.Session: chapter_defs: vs. known_chapters (in declaration order) vs. chapters (covered by visible sessions); clarified browser_info root index: preserve declaration order as much as possible;

clarified syntax: more uniform;

more robust, more strict;

tuned signature;