a factor with levels Alabama,
Alaska, Arizona, Arkansas, California,
Colorado, Connecticut, Delaware, District of
Colunbia, Florida, Georgia, Hawaii, Idaho,
Illinois, Indiana, Iowa, Kansas, Kentucky,
Louisiana, Maine, Maryland, Massachusetts,
Michigan, Minnesota, Mississippi, Missour,
Montana, Nebraska, Nevada, New Hampshire, New
Jersey, New Mexico, New York, North Carolina, North
Dakota, Ohio, Oklahoma, Oregon, Pennsylvania,
Rhode Island, South Carolina, South Dakota,
Tennessee, Texas, Utah, Vermont, Virginia,
Washington, West Virginia, Wisconsin, and Wyoming