Involutory modules and involutory differential algebras

The paper deals with algebraic models of differential geometry. Prolongations of exterior differential algebras defined by exact sequences of free graduated modules over exterior algebra are considered. A graduated module is called involutory in a sense generalizing the use of this term by E. Cartan and Spencer.
Using the notion of involutory graduated module, a criterion is found for the existence of an infinite sequenre of prolongations of a given differential algebra.