“Every weeknight, Alvin Ailey holds classes for the general public in ballet, hip-hop, Middle Eastern dance and other dance forms, including, on Friday, Afro-Cuban, the swaying dance and ecstatic music of the plantation workers of Cuba. It is an eye-catching performance at the corner of Ninth Avenue and 55th Street, and passers-by — sometimes one, sometimes 30 — take in the free show.”