 Complete the following phrase:  The Polikarpov I-16  is a Soviet single-engine single-seat fighter aircraft of revolutionary design; it was the worlds first low-wing cantilever monoplane fighter with retractable landing gear to attain operational status and as such introduced a new vogue in fighter design. The I-16 was introduced in the mid-1930s and formed the backbone of the Soviet Air Force at the beginning of World War II. The diminutive fighter, nicknamed Ishak or Ishachok donkey or burro by Soviet pilots, figured prominently in the Second Sino-Japanese War, the 