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;
#optional components: somewhat experimental tools
hol-light-bundle-0.5-126