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I would strongly suggest you take a look at Band-In-A-Box. It is a software program that runs on your computer and provides a backup band to match the melody and chords you enter. I feel it is better suited to songwriting than an aranger keyboard. It also has a great other many features that horn players will appreciate.
Google on "PG Music." Check out their site, especially the user forum. Download the demo and try it out. Please report back what you decide.
Douglas Wolfe
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