WASHINGTON (Reuters) -The U.S. Supreme Court is set on Monday to consider the legality of a provision of the Obamacare law, formally called the Affordable Care Act, that helps ensure that health insurers cover preventive medical care such as cancer screenings at no cost to patients. The federal government has appealed a lower court's determination that the U.S. Preventive Services Task Force, which under Obamacare has a major hand in choosing what services will be covered, is composed of members who were not validly appointed. Its 16 members are appointed by the U.S. secretary of health and human services without Senate confirmation.