What is the 1981 value at the Tournament of Wimbledon?
CREATE TABLE "grand_slam_singles_tournament_timeline" (
  "tournament" text,
  "1980" text,
  "1981" text,
  "1982" text,
  "1983" text,
  "1984" text,
  "1985" text,
  "1986" text,
  "1987" text,
  "1988" text,
  "1989" text,
  "1990" text,
  "1991" text,
  "1992" text,
  "1993" text,
  "1994" text,
  "1995" text,
  "1996" text
);
SELECT "1981" FROM "grand_slam_singles_tournament_timeline" WHERE "tournament"='wimbledon';