State immunity — Jurisdictional immunity — State corporation — Commercial activity exception — Distinction between sovereign and commercial acts — Distinction turning on nature, rather than purpose of act — Mexican Government owning majority shareholding in Mexican corporation — Corporation providing transportation at behest of Mexican Government for university officials wishing to conduct government-sponsored research in Mexico — Whether provision of transport constituting a commercial activity — Requirement that commercial activity having substantial contact with United States — Arrangement of return journey from the United States to Mexico — Whether constituting substantial contact — United States Foreign Sovereign Immunities Act 1976 — The law of the United States