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The killing of the al-Qaeda mastermind didn’t undo any of the changes Washington has seen post-Sept. 11: The Jersey barriers are still up and the ambient terror remains. But the night of bin Laden’s death did demonstrate one unrelated phenomenon: In Washington, celebrating in Lafayette Square has become the standard response to joyous national events. In 2008, student crowds took to the square after Barack Obama’s victory. This year, similar revelers did so to mark Osama’s demise. (Bikeshare rentals spiked just after the late-Sunday-night announcement of the killing.) And now that TV camera crews know it, the party in front of the White House will likely become institutionalized: Not only is it a chance to whoop it up, but revelers can get on the news, too.