Analogue-sequencer

>  So auxD = the D accumulator,
> whatever accumulator is active in the D slot?  It's just a raw integer
> value under the hood, regardless of what it's accumulating?

Aux D is the fourth aux after A, B anc C.
The patterns hold a value for each aux on every step.

The Aux D accumulator is a persistent value, which is added to the step
value for aux D on every step.
All three accumulators have a range of -128 to +127.
There is an option to 'disconnect' the Aux D accumulator, so you can use aux
D for un-accumulated events or CCs, while still using aux D masking.
 
> Also, if I haven't explicitly activated an accumulator in D, is it
> just treated as a zero value?

The accumulators are always active, but they are reset to zero whenever P3
is started or a new pattern selected, so unless you explicitly set up an
event to alter the accumulator value, it won't have any effect.

> Which sort of leads me to my next question, once I've got a
> complicated sequence running with a bunch of accumulators, masks,
> randomize events etc, it can get pretty tricky to unroll what's going
> on, particularly if I come back to the P3 after a while.  Any tips for
> keeping things straight or is it just a question of keeping focused
> and paying close attention?  There's no way to temporarily mute some
> or all aux events or anything like that, right?

Once you get the hang of what the events do, you can pretty quickly work out
what's going on by stepping through the four auxes in a pattern you haven't
looked at for a while.
Sometimes it can be good not to be entirely sure what's going on ;-)

Best regards,
Colin Fraser
Sequentix Music Systems Ltd
http://www.sequentix.com