code: func (v Vec3) Dist2D(v1 Vec3) float32 {
  	dx := v1[0] - v[0]
  	dz := v1[2] - v[2]
  	return math32.Sqrt(dx*dx + dz*dz)
  }
docs: // Dist2D derives the distance between v and v2 on the xz-plane.
  //
  // The vectors are projected onto the xz-plane, so the y-values are ignored.