Question: The Advanced Tactical Fighter (ATF) was a demonstration and validation program undertaken by the United States Air Force, including the Soviet Sukhoi Su-27, a twin-engine supermaneuverable fighter aircraft designed by Sukhoi, it was designed to complement which aircraft?
Context: The Lockheed/Boeing/General Dynamics YF-22 was an American single-seat, twin-engine fighter aircraft technology demonstrator designed for the United States Air Force (USAF).  The design was a finalist in the USAF's Advanced Tactical Fighter competition, and two prototypes were built for the demonstration/validation phase of the competition.  The YF-22 won the contest against the Northrop YF-23, and entered production as the Lockheed Martin F-22 Raptor.  The YF-22 has similar aerodynamic layout and configuration as the F-22, but with differences in the position and design of the cockpit, tail fins and wings, and in internal structural layout. The Sukhoi Su-27 (Russian: Сухой Су-27 ; NATO reporting name: Flanker; nicknamed "Sushka" in USSR) is a twin-engine supermaneuverable fighter aircraft designed by Sukhoi.  It was intended as a direct competitor for the large United States fourth-generation fighters such as the Grumman F-14 Tomcat and F-15 Eagle, with 3530 km range, heavy aircraft ordnance, sophisticated avionics and high maneuverability.  The Su-27 was designed for air superiority missions, and subsequent variants are able to perform almost all aerial warfare operations.  It was designed with the Mikoyan MiG-29 as its complement. The Northrop/McDonnell Douglas YF-23 was an American single-seat, twin-engine stealth fighter aircraft technology demonstrator designed for the United States Air Force (USAF).  The design was a finalist in the USAF's Advanced Tactical Fighter (ATF) competition, battling the Lockheed YF-22 for a production contract.  Two YF-23 prototypes were built, nicknamed "Black Widow II" and "Gray Ghost". The Lockheed Martin F-22 Raptor is a fifth-generation, single-seat, twin-engine, all-weather stealth tactical fighter aircraft developed for the United States Air Force (USAF).  The result of the USAF's Advanced Tactical Fighter program, the aircraft was designed primarily as an air superiority fighter, but also has ground attack, electronic warfare, and signal intelligence capabilities.  The prime contractor, Lockheed Martin, built most of the F-22's airframe and weapons systems and conducted final assembly, while Boeing provided the wings, aft fuselage, avionics integration, and training systems. The Mikoyan MiG-29 (Russian: Микоян МиГ-29 ; NATO reporting name: Fulcrum) is a twin-engine jet fighter aircraft designed in the Soviet Union.  Developed by the Mikoyan design bureau as an air superiority fighter during the 1970s, the MiG-29, along with the larger Sukhoi Su-27, was developed to counter new American fighters such as the McDonnell Douglas F-15 Eagle, and the General Dynamics F-16 Fighting Falcon.  The MiG-29 entered service with the Soviet Air Force in 1982. The General Electric YF120 was a variable cycle turbofan engine designed by GE Aircraft Engines in the late 1980s/early 1990s for the United States Air Force's Advanced Tactical Fighter (ATF) project (which resulted in the F-22 Raptor).  GE lost the engine competition for this aircraft to Pratt & Whitney F119. The AL-41F is a designation for two different Russian military turbofan engine variants.  The NPO Saturn AL-41F is a Russian variable-bypass ratio turbofan engine, designed for supercruise flight for the "MFI" ("Mnogofunktsionalni Frontovoy Istrebitel", "Multifunctional Frontline Fighter") program, which resulted in the Mikoyan Project 1.44.  It is considered by Jane's as the Russian counterpart to the General Electric YF120 engine which lost to the more conventional fixed-bypass YF-119 in the Advanced Tactical Fighter engine program.  Since the cancellation of the MFI program, the AL-41F1S and AL-41F1 designation was assigned to heavily upgraded AL-31F variants that powers the Su-35S and initial production Sukhoi Su-57 stealth aircraft. The McDonnell Douglas F-15 Eagle is an American twin-engine, all-weather tactical fighter aircraft designed by McDonnell Douglas (now Boeing) to gain and maintain air supremacy in aerial combat.  Following reviews of proposals, the United States Air Force selected McDonnell Douglas' design in 1967 to meet the service's need for a dedicated air superiority fighter.  The Eagle first flew in July 1972, and entered service in 1976.  It is among the most successful modern fighters, with over 100 victories and no losses in aerial combat, with the majority of the kills scored by the Israeli Air Force. The Advanced Tactical Fighter (ATF) was a demonstration and validation program undertaken by the United States Air Force to develop a next-generation air superiority fighter to counter emerging worldwide threats, including Soviet Sukhoi Su-27 and Mikoyan MiG-29 fighters under development in the 1980s.  Lockheed and Northrop were selected in 1986 to develop the YF-22 and the YF-23 technology demonstrator aircraft.  These aircraft were evaluated in 1991 and the Lockheed YF-22 was selected and later developed into the F-22 Raptor. The Sukhoi Su-57 (Russian: Сухой Су-57 ) is the designation intended for a stealth, single-seat, twin-engine jet multirole fighter aircraft designed for air superiority and attack operations.  The aircraft is the product of the PAK FA (Russian: ПАК ФА , Russian: Перспективный авиационный комплекс фронтовой авиации , "Perspektivny Aviatsionny Kompleks Frontovoy Aviatsii", literally "Prospective Airborne Complex of Frontline Aviation"), a fifth-generation fighter programme of the Russian Air Force.  Sukhoi's internal name for the aircraft is T-50.  The Su-57 will be the first aircraft in Russian military service to use stealth technology.  The fighter is planned to have supercruise, stealth, supermaneuverability and advanced avionics to overcome the prior generation fighter aircraft as well as ground and maritime defences.
Answer: Mikoyan MiG-29