You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fft.go
    path of file: ./repos/sliver/vendor/github.com/remyoudompheng/bigfft
    
    the code of the file until where you have to start completion: // Package bigfft implements multiplication of big.Int using FFT.
//
// The implementation is based on the Schönhage-Strassen method
// using integer FFT modulo 2^n+1.
package bigff
    
func (p *poly) Mul(q *poly) poly {
	// extra=2 because:
	// * some power of 2 is a K-th root of unity when n is a multiple of K/2.
	// * 2 itself is a square (see fermat.ShiftHalf)
	n := valueSize(p.k, p.m, 2)

	pv, qv := p.Transform(n), q.Transform(n)
	rv := pv.Mul(&qv)
	r := rv.InvTransform()
	r.m = p.m
	return r
}