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use HOME and END keys to cycle through previous and next unit, respectively. To cycle through all units and cities together, use the TAB key.
Will there also be a pair of keys for sites so you can step forward and backward through just them like HOME and END do for units?
p.s. It might also be good to assign SHIFT+TAB as the opposite direction on whatever list TAB steps through. It's sort of a logical opposite, and having HOME and END work as a bidirectional pair leaves me expecting the same thing for other lists.