What is the sum of points in 1991, for footwork a11c chassis?
CREATE TABLE "complete_formula_one_results" (
  "year" real,
  "entrant" text,
  "chassis" text,
  "engine" text,
  "points" real
);
SELECT SUM("points") FROM "complete_formula_one_results" WHERE "year"=1991 AND "chassis"='footwork a11c';