State immunity — Jurisdictional immunity — Claim for damages for personal injuries sustained in car accident — Standard of proof to be applied when determining jurisdiction — Whether stricter standard of proof required where facts go to the merits as well as to jurisdiction and are disputed — Court's discretion to determine method to resolve disputed facts for purposes of determining jurisdiction — Foreign Sovereign Immunities Act 1976 — Tortious activity exception to immunity — Accident caused by car driven by foreign serviceman — Whether journey in private car to attend doctor's appointment within scope of employment — The law of the United States