State immunity — Jurisdictional immunity — Commercial activity exception — Burden of proof — Assistance Agreement between the United States and the Somali Democratic Republic — Shipment of grain to Somalia by United States company — Payment demanded by Somali State corporation for detention of vessel — Whether Somali Democratic Republic or State corporation entitled to sovereign immunity — Connection with the United States — Waiver of immunity — United States Foreign Sovereign Immunities Act 1976 — The law of the United States