I was confused about what talking about back in 2004, Frank. I understand the concept, but just didn't get at first that was what you meant. This is because one can't skim thru explanations of music theory.

As to the A7, Artful Codger, it's not a "harmonic alternative", it is the dominant chord in the key of D minor, so when you move from C to A7, you've got a key change. The trick here is that the D minor chord is also the ii chord, and an extension of the Dominant harmony, in the key C(Dm: D F A, G9: G B D F A), so depending on where you go next from the Dm, you can be in either the key of D minor or the key of C.