State immunity — Head of State’s immunity — Whether applicable to Head of State when not de facto ruler of State — Claim for compensation for judicial killing against exiled ruler recognized as lawful Head of State by United States Executive — Suggestion of immunity letter issued by Executive — Whether court compelled to defer to determination by Executive — Whether implied waiver of immunity effective — Whether United States Foreign Sovereign Immunities Act 1976 or Torture Victim Protection Act 1992 can negate immunity — The law of the United States