  Many methods for reducing and simplifying differential equations are known.
They provide various generalizations of the original symmetry approach of
Sophus Lie. Plenty of relations between them have been noticed and in this note
a unifying approach will be discussed. It is rather close to the classical
differential constraint method, but we provide certain rigorous results basing
on recent advances in compatibility theory of non-linear overdetermined systems
and homological methods for PDEs.
