Who vacated his post when his successor was formally installed on May 11, 1966?
CREATE TABLE "senate" (
  "state_class" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_of_successors_formal_installation" text
);
SELECT "vacator" FROM "senate" WHERE "date_of_successors_formal_installation"='May 11, 1966';