State immunity — Head of State immunity — Whether former President of Philippines and his wife entitled to Head of State immunity — Scope of immunity — Whether customary international law existing — Restrictive theory of immunity — Foreign Sovereign Immunities Act 1976 — Whether appropriate for court to decide whether Head of State entitled to immunity — Whether Head of State immunity capable of being waived — Whether Philippine Government waiving immunity — Related doctrines of foreign sovereign immunity and diplomatic immunity — Purpose of doctrines — Mutual respect and comity among foreign States — The law of the United States of America