What's the inception for the country with 25.5 billion USD in assets and an abbreviation of psf?
CREATE TABLE "largest_sovereign_wealth_funds" (
  "country" text,
  "abbreviation" text,
  "assets_us_billion" text,
  "inception" text,
  "origin" text
);
SELECT "inception" FROM "largest_sovereign_wealth_funds" WHERE "assets_us_billion"='25.5' AND "abbreviation"='psf';