How many reasons were given when A. Willis Robertson (D) resigned?
CREATE TABLE "senate" (
  "state_class" text,
  "vacator" text,
  "reason_for_change" text,
  "successor" text,
  "date_of_successors_formal_installation" text
);
SELECT COUNT("reason_for_change") FROM "senate" WHERE "vacator"='A. Willis Robertson (D)';