  1) The differential equation considered in terms of exterior differential
forms, as \'E.Cartan did, singles out a differential ideal in the
supercommutative superalgebra of differential forms, hence an affine
supervariety. In view of this observation, it is evident that every
differential equation has a supersymmetry (perhaps trivial). Superymmetries of
which (systems of) classical differential equations are missed yet?
  2) Why criteria of formal integrability of differential equations are never
used in practice?
