f64/d2/vec.go
func (v Vec) Len() float64 {
	return float64(math.Hypot(float64(v.X), float64(v.Y)))
}
func (v Vec) Len() float64 {
	return float64(math.Hypot(float64(v.X), float64(v.Y)))
}
// Len returns the vector's length.