Fugazi have earned a reputation for being severe, but their lighter side often emerges on instrumentals, like the motorik guitar odyssey “Arpeggiator,” or the string-plucked post-rock prototype “Sweet and Low.” And by 2001's The Argument, they had become an insidiously catchy power pop band: “Nightshop” is an exhilarating joyride of a song that yields one of Guy Picciotto's sharpest hooks.