It's a clear Sunday afternoon in Dolores Park, but Matt, 26, is on alert. He just sold a cold drink to a picnicker, something mixed and alcoholic, in a tall plastic cup. To pay him, she scans a QR code on the laminated sheet of paper. Now, while she lies back and enjoys her first sips, Matt cranes his neck and peers over the hill. A park ranger is making the rounds. If she catches him, he could be asked to leave the park. Without peeling his eyes away from the hill, he makes a phone call.