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)