  On any manifold, any non-degenerate symmetric 2-form (metric) and any
skew-symmetric (differential) form W can be reduced to a canonical form at any
point, but not in any neighborhood: the respective obstructions being the
Riemannian tensor and dW. The obstructions to flatness (to reducibility to a
canonical form) are well-known for any G-structure, not only for Riemannian or
symplectic structures.
  For the manifold with a nonholonomic structure (nonintegrable distribution),
the general notions of flatness and obstructions to it, though of huge interest
(e.g., in supergravity) were not known until recently, though particular cases
were known for more than a century (e.g., any contact structure is ``flat'': it
can always be reduced, locally, to a canonical form).
  We give a general definition of the NONHOLONOMIC analogs of the Riemann and
Weyl tensors. With the help of Premet's theorems and a package SuperLie we
calculate these tensors for the particular case of flag varieties associated
with each maximal (and several other) parabolic subalgebra of each simple Lie
algebra. We also compute obstructions to flatness of the G(2)-structure and its
nonholonomic super counterpart.
