Golang prepend implementation

prepend.go

type Xs struct {
	xs  []*X
	idx int
}

func (xs *Xs) prepend(x *X) *Xs {
	if xs.idx == 0 {
		ys := make([]*X, (len(xs.xs)+1)*2)
		xs.idx = len(xs.xs) + 1
		copy(ys[xs.idx+1:], xs.xs)
		ys[xs.idx] = x
		xs.xs = ys
	} else {
		xs.idx--
		xs.xs[xs.idx] = x
	}
	return xs
}

func (xs *Xs) slice() []*X {
	return xs.xs[xs.idx:]
}