Summary:Note: All the following instructions are intended to help you understand the template and are not guaranteed to be correct.Some pitfalls: the range of fft, whether the array is empty, how many powers of module x (all polynomials in the same formula must be polynomials in the sense of the same module $x ^ n $), the polynomial inversion is known $A (x) $, and the B satisfying $A (x) B (x)=1 (mod x ^ n) $(let's assume that n is a power of 2 for convenience) is multiplied,read the whole passage
