Beethoven's harmonic web

I was thinking about harmonic modulations in music and how it is a metaphor for the way the brain works: It makes a circuitous path, building neurons along the way. Beethoven would weave seamlessly in and out of keys and you wouldn't even know it, yet would arrive in a completely different world (key).

Even though functional harmony has almost completely devolved out of music, it is no less interesting to examine.

This is how Beethoven would handle a modulation from D Major ( 2 sharps) to B-Flat Minor (5 flats) and back again:

D / A7b9 / D- / F /F7 / Bb- / A7