more errors;

clarified inversion of file name to theory name, notably for Windows;

clarified import_name: observe directory notation more strictly;

tuned signature;

clarified signature -- removed pointless operations;

clarified signature -- removed unused content;

clarified theory imports completion, based on session directories and current master directory (no support for local session-subdirectories);

find theories via session directories only -- ignore known_theories;

tuned;

tuned message;