clarified directory layout: files are relative to enclosing theory;

tuned signature: avoid duplication;

more robust: theories could have been suppressed via option "condition";

tuned signature;

tuned messages (again, see d50c2129e73a): presentation setup could fail initially for take some time;

clarified modules;

clarified signature: support for adhoc file types;

clarified Presentation.Nodes, with explicit Nodes.Session and Nodes.Theory; distinguish Nodes.Theory.static_session vs. dynamic_session: refer to exports from dynamic_session, corresponding to strictly to build_graph; more robust treatment of source files and links to generated files; retrieve entities by their file position, within its corresponding session/theory hierarchy;

export entity file position as well, e.g. relevant for HTML presentation with aux. files;

proper permissive = true (amending 475fedc02737)

tuned signature;

clarified signature;

clarified signature;

more robust directory structure: always relative to session_dir; tuned messages;

discontinued slightly odd integrity check (from af2d0e07493b): requires a different approach;

clarified signature;

misc tuning and clarification;

unused;

tuned signature;

tuned signature;

tuned signature;

clarified modules;

clarified modules;

unused;

tuned signature;

clarified signature: replaced Sessions.Deps by Sessions.Structure from HTML_Context;

tuned;

clarified signature: avoid constants from Sessions.Structure within Session.Base;

clarified signature: avoid object-oriented HTML_Context; clarified theory_qualifier --- belongs to the overall Sessions.Structure;

tuned type signature