Which episodes did Nelson McCormick direct?
CREATE TABLE "table1_17355820_1" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text
);
SELECT "written_by" FROM "table1_17355820_1" WHERE "directed_by"='Nelson McCormick';