code: func (u Vec) Map(f func(float64) float64) Vec {
  	return Vec{
  		f(u.X),
  		f(u.Y),
  	}
  }
docs: // Map applies the function f to both x and y components of the vector u and returns the modified
  // vector.
  //
  //   u := pixel.V(10.5, -1.5)
  //   v := u.Map(math.Floor)   // v is Vec(10, -2), both components of u floored