ROOTS

clarified signature selection: SortedSet[String], which fits better to stored json and works properly on Windows (NB: document theories have an authentic session-theory name);

src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools