State immunity — Jurisdictional immunity — Commercial activity — Implied waiver of immunity — Writ of garnishment on assets of foreign State entity to satisfy judgment debt in action arising from an agreement to which State entity was not a party — Agreement with petroleum workers trade union — Writ of garnishment on assets of national oil company — Whether trades union could be regarded as agent of national oil company — Whether provision in statute creating foreign State entity permitting it “to sue and be sued” constitutes a waiver of immunity — The law of the United States