which offers a variety of products (bake shop, mail order, creamery, coffee) and services (business education). Flint Ink Corp., another Ann Arbor-based company, was the world's largest privately held ink manufacturer until it was acquired by Stuttgart-based XSYS Print Solutions in October 2005. Avfuel, a global supplier of aviation fuels and services, is also headquartered in Ann Arbor. Aastrom Biosciences, a publicly traded company that develops stem cell treatments for cardiovascular diseases, is also headquartered in Ann Arbor. Many cooperative enterprises were founded in the city; among those that remain are the People's Food Co-op and the Inter-Cooperative Council at the University of Michigan, a student housing cooperative founded in 1937. There are also three cohousing communities—Sunward, Great Oak, and Touchstone—located immediately to the west of the city limits. Culture Several performing arts groups and facilities are on the University of Michigan's campus, as are museums dedicated to art, archaeology, and natural history and sciences. Founded in 1879, the University Musical Society is an independent performing arts organization that presents over 60 events each year, bringing international artists in music, dance, and theater. Since 2001 Shakespeare in the Arb has presented one play by Shakespeare each June, in a large park near downtown. Regional and local performing arts groups not associated with the university include the Ann Arbor Civic Theatre, the Arbor Opera Theater, the Ann Arbor Symphony Orchestra, the Ann Arbor Ballet Theater, the Ann Arbor Civic Ballet (established in 1954 as Michigan's first chartered ballet company), The Ark, and Performance Network Theatre. Another unique piece of artistic expression in Ann Arbor is the fairy doors. These small portals are examples of installation art and can be found throughout the downtown area. The Ann Arbor Hands-On Museum is located in a renovated and expanded historic downtown fire station. Multiple art galleries exist in the city, notably in the downtown area and around the University of Michigan campus. Aside from a large restaurant scene in the Main Street, South State Street, and South University Avenue areas, Ann Arbor ranks first among U.S. cities in the number of booksellers and books sold per capita. The Ann Arbor District Library maintains four branch outlets in addition to its main downtown building. The city is also home to the Gerald R. Ford Presidential Library. Several annual events—many of them centered on performing and visual arts—draw visitors to Ann Arbor. One such event is