Like many politicians running for office, Zohran Mamdani, the 33-year-old New York Democratic mayoral candidate, is partly a fabulist. His story includes tax hikes on the wealthy and businesses, free buses, free child care and a $30 minimum wage by 2030. Whether or not these are good ideas, most have as little likelihood of becoming tangible as one of Italo Calvino’s odd, enchanting, invisible cities.