code: func (v Vec3) Perp2D(u Vec3) float32 {
  	return v[2]*u[0] - v[0]*u[2]
  }
docs: // Perp2D derives the xz-plane 2D perp product of the two vectors. (uz*vx - ux*vz)
  //
  // u is the LHV vector [(x, y, z)]
  // v is the RHV vector [(x, y, z)]
  //
  // The vectors are projected onto the xz-plane, so the y-values are ignored.