What was found
AT FL 200, THE NR 2 ENGINE ROLLED BACK SUDDENLY. WARNING LIGHTS BUT NO HORNS. STARTED A DESCENT, TURNED OFF AUTO THROTTLES AND SELECTED CONTINOUS RELIGHT AFTER CHECKING N1 AND N2. THE ENGINE RESPONDED, ELECTED TO LEAVE IT AT IDLE UNTIL THE THRUST REVERSERS. DECLARED EMERGENCY. SET UP FOR 20 FLAP LANDING AND RAN THE APPROPRIATE PARTS OF THE SINGLE ENGINE OPERATION CHECKLIST. VERY SMOOTH LANDING JUST BELOW MAX LANDING GROSS WEIGHT. FADEC ALERT ON ENGINE 2 DURING TAXI IN. REPLACED NR 2 ENG EEC PER MM 73-21-01-04. BITE TEST AND GROUND RUN GOOD. PERFORMED 5 MIN IDLE AND 5 MIN POWER RUN AT CRUISE POWER PER JRH. NO DEFECTS NOTED.