analogue of the Nijenhuis tensor, the obstruction to the integrability of an almost real-
complex structure. The Nijenhuis tensor vanishes identically only if the superstring
superdimension is 1| 1 and, moreover, the superstring is endowed with a contact structure.
We also show that all real forms of Grassmann algebras are isomorphic, although they are
defined by obviously different anti-involutions.