Who is the writer of episode 13?
CREATE TABLE "table1_2226817_8" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text
);
SELECT COUNT("written_by") FROM "table1_2226817_8" WHERE "no_in_season"=13;