What is the fine-grained topic of the following text: Painted over three exceedingly chilly days in November, his rooftop mural masterpiece, titled “Ceci N’est Pas Une Grosse Pomme” ("This Is Not a Big Apple”), features RO’s signature interlocking calligraphy in zen-like black strokes on white. Only return the topic.
RO's rooftop mural masterpiece "Ceci N’est Pas Une Grosse Pomme"