Which leader has been a leader since 2007?
CREATE TABLE "leaders_in_the_northern_ireland_assembel" (
  "leader" text,
  "party" text,
  "leader_s_seat" text,
  "number_of_seats" real,
  "leader_since" text
);
SELECT "leader" FROM "leaders_in_the_northern_ireland_assembel" WHERE "leader_since"='2007';