Thanks, Eddy. That's interesting. How I see the music is...
1. Use of a pedal point Bb in the first three bars.
2. The A note in the soprano, bar 1, looks like it's a prepared dissonance since it initially occurs as a consonance that's carried over from the anacrusis.
I see the first chord as Bbmb5 (Bbdim triad) with the A as a dissonance (as mentioned above) that resolves to the G in the following bar. Diminished triads are supported in BIAB. If you have it set up, you can simply type Xdim, otherwise use Xdim5. (It's under Display Options.)
Because of the pedal point, it's also possible that Bb in the bass is the dissonance that resolves downwards to A in bar 4. A diminished chord is not a usual way to begin pedal point but, if it works, it works.
Possible chord progressions that I'd try...
If Bb (bass) is treated as the dissonance and the Db is treated as C#, it's enharmonic equivalent..
ONEA/Bb - Dbdim/Bb - Cm/Bb - F7/A
If A (soprano) is treated as the dissonance, then...
TWOBbdim (triad) - Bbdim (triad) - Cm/Bb - F7/A
To get the best pedalpoint effect for the above, it would be possible to use "Chord Settings / Pedal Bass".
(Alternatively, try Bbdim - Dbdim/Bb - Cm/Bb - F7/A.)
I suspect that the first option under "TWO" will work best. I wouldn't worry about the soprano notes (A and then G) because these are melody and will either be sung or played and, when that happens, they will provide the required harmonic sound. I suspect that Bbdim will provide sufficient textural backing for the first two bars.
...MGU file with my suggestionIn both cases, I'd use Cm/Bb and not Cm7/Bb since the second option would double the 7th and this would most likely sound overdone.
That's just my take on it... it will be interesting to see what others say.
Regards,
Noel