Jean Nouvel's supertall tower at 53 West 53rd Street is really, truly becoming a reality, after many years of waiting, and today the Times has the first look inside the building. The interior rendering shows a pretty grand (but familiar) view of Central Park, but it also highlights the building's unusual diagonal structural system, known as a "diagrid." This gives the tower its signature zig-zagging pattern on the facade, but inside, it causes a lot of design headaches, including tilting windows and slanted columns. For interior designer Thierry W. Despont to work out all of the kinks, developer Hines opted to build a full-scale model unitlikened to a "playground," "a lab" or one big "toy"in a warehouse in Sunset Park. The prototype cost $500,000 to $1 million to build, but that's barely a drop in the bucket considering the 140 condos will list for $3 million to more than $50 million.