State immunity — Jurisdictional immunity — Commercial activity exception — Distinction between sovereign and commercial acts — Distinction turning on nature rather than purpose of act — Lack of profit motive — Whether creating presumption of sovereign act — Greece — Constitutional obligation to provide health care for citizens — Contract to provide kidney transplants for Greek citizens in United States hospitals — Whether entering into contract a commercial activity — Whether courts required to determine if commercial activity undertaken in the United States — Foreign Sovereign Immunities Act 1976 — The law of the United States