Name the mixed doubles for year before 1991 and men's doubles of david jepson buck ryan
CREATE TABLE "winners" (
  "year" real,
  "men_s_singles" text,
  "women_s_singles" text,
  "men_s_doubles" text,
  "women_s_doubles" text,
  "mixed_doubles" text
);
SELECT "mixed_doubles" FROM "winners" WHERE "year"<1991 AND "men_s_doubles"='david jepson buck ryan';