About 200 angry veterans, their friends and families, some choking back tears, marched on the Art Institute in Chicago, demanding an end to a student exhibit that allows the American flag to be walked upon. “We are absolutely outraged at the insensitivity of the Art Institute,” Tom Stack, a Vietnam veteran, told flag- and sign-waving protesters from the steps of the institute. In subfreezing, windy weather, Stack told the crowd, including several veterans in wheelchairs, that the issue is respect for the nation’s symbol, not freedom of expression. The exhibit features the flag spread on the floor beneath photographs of flags, including some draped over coffins, and a shelf which holds a ledger. Visitors may use the ledger to record their comments.