'Derivation' property

A 'Derivation' should be an object which non-commutes with everything that 'Depends' on it, and for which @prodrule gives

D a b c -> (D a) b c + a (D b) c + a b (D c)

Moreover, '@prodflatten' should never take the brackets away in these situations.

It would be good to have a solution in mind which can easily be extended to work with graded algebras, so that exterior derivatives can be handled with the same logic.