Yes . I think some form of auto accompaniment must be included in the definition Cgiles. I dont think that an arranger keyboard can be defined by how its used because thats simply too wide a definition.Literally anything could be an arranger.
Of the list that you put up genesys i think the quality of the onboard styles and specific arranger features like being able to drop in and out parts on a style are essential for any aranger.
I agree with most of wht you say about the way that arrangers can be used that are not the way that they are traditionally used. For examle I use my instrument to specifically rearrange established songs in a new way.I hardly ever play solos in the right hand with a different instrument to what i have in the left hand.
Your example of the mediastation might be contentious as noone has a problem with the instrument as a concept. The controversy comes about whether or not it does what its supposed to do to a standard that people generally accept as quality. I dont want to dwell on this . I just wanted to clarify that understanding the concept of new arrangers is not the problem.
"I think that an arranger is a keyboard that gives one person the ability to at most sound like a ban with the flexibility of being spontaneous on the fly. "
I completely agree.
I think the problem with XS is that the instrument is not specifically designed for arranger use although it has some limited aranger functions. I think the instruments primary function should be for arranger use and that kind of makes it an arranger.
Good luck if you can find a tighter definition .
Not to mimic the original but change it
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dont quit.......period