\Bibitem{Tol48} \by G.~P.~Tolstov \paper On the total differential \jour Russian Math. Surveys \yr 1948 \vol 3 \issue 5 \mathnet{http://mi.mathnet.ru//eng/rm8762} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=27044} \zmath{https://zbmath.org/?q=an:0032.34301}