Must armrests in exit seats must be fixed, or is it up to the operator to decide whether the armrests can be of the movable type?

In all my travels on airlines around the world, I’ve yet to see an exit seat with movable armrests. I’m not 100% sure if there is a regulation that requires it, however from a functional standpoint, all exit seat armrests are fixed because they contain the tray used by a passenger seated there. 

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