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Let's say you want to invoke a program from HUD, you can do that too. Here, I'm getting ready to invoke my e-mail program to send a message.
Last, but not least, you can also HUD to select files—in this case an album—from within a program. The idea, as I'm sure you figured out by now, is to use the same interface for both the desktop at large and all its applications. Eventually, Ubuntu plans to use the exact same HUD interface for tablets and smartphones as well.