What Social AO has an External CO of elena buscher, and a President of harm van leeuwen?
CREATE TABLE "past_composition_of_the_executive_board" (
  "president" text,
  "treasurer" text,
  "secretary" text,
  "social_ao" text,
  "academic_ao" text,
  "internal_co" text,
  "external_co" text
);
SELECT "social_ao" FROM "past_composition_of_the_executive_board" WHERE "external_co"='elena buscher' AND "president"='harm van leeuwen';