  The classical theory of prolongation of G-structures was generalized by N.
Tanaka to a wide class of geometric structures (Tanaka structures), which are
defined on a non-holonomic distribution. Examples of Tanaka structures include
subriemannian, subconformal, CR-structures, structures associated to second
order differential equations and structures defined by gradings of Lie algebras
(in the setting of parabolic geometry). Tanaka's prolongation procedure
associates to a Tanaka structure of finite order a manifold with an absolute
parallelism. It is a very fruitful method for the description of local
invariants, investigation of the automorphism group and the equivalence
problem. In this paper we develop an alternative constructive approach for
Tanaka's prolongation procedure, based on the theory of quasi-gradations in
filtered vector spaces, G-structures and their torsion functions.
