What PRR class has a Wheel arrangement of a1a-a1a?
CREATE TABLE "emd" (
  "prr_class" text,
  "builder_s_model" text,
  "build_date" text,
  "total_produced" real,
  "wheel_arrangement" text,
  "service" text,
  "power_output" text
);
SELECT "prr_class" FROM "emd" WHERE "wheel_arrangement"='a1a-a1a';