F mul_inv(=a, =b)
   V b0 = b
   V x0 = 0
   V x1 = 1
   I b == 1 {R 1}

   L a > 1
      V q = a I/ b
      (a, b) = (b, a % b)
      (x0, x1) = (x1 - q * x0, x0)

   I x1 < 0 {x1 += b0}
   R x1

print(mul_inv(42, 2017))