It's been seven years since Mark Matos came to S.F., and he's had a good run. Together with what has sometimes seemed like every musician in the Mission, Matos led his loosely organized rock group, Os Beaches, and a startling number of musical side projects through straight-ahead shows and off-the-wall experiments. Crossing through territories of folk, rock, and even jazz, Matos, aka Trans Van Santos, has helped wave the flag for freaky music in San Francisco. Alas: he too is now leaving, to build a home and a studio in Joshua Tree. "We want different things, San Francisco and I," Matos says. " It's time to move on, it's time to get going." But not without one last goodbye. This week, Matos assembles a "Last Waltz in Little China" — one final Os Beaches show with friends and special guests to bid farewell to the city, and to celebrate a new box set of demos, rarities, maps, and artifacts called How to Survive in California. Like most things Matos, this last dance promises to be loose, heartfelt, and emphatically unburdened by convention.