anchor.diff:  [stem]
   ++++
   \begin{align*}
  -\vec{v'}&={q_1}\vec{v}{q_1^{-1}}
  -\vec{v''}&={q_2}\vec{v'}{q_2^{-1}}
  +\vec{v'}&={q_1}\vec{v}{q_1^{-1}}\\
  +\vec{v''}&={q_2}\vec{v'}{q_2^{-1}}\\
   &={q_2}{q_1}\vec{v}{q_1^{-1}}{q_2^{-1}}\\
   &=({q_2}{q_1})\vec{v}({q_2}{q_1})^{-1}\\
   \end{align*}
  
positive.code: 
  ---
  [NOTE, indent=2]
  .Note
  ====
  We can however get rid of the negative sign by choosing stem:[\vec{a}] and stem:[\vec{b}] differently.
  
  
negative.code: 
  ---
  [NOTE, indent=2]
  .Side Note
  ====
  This claim actually has 2 sides. Let me go a little be more in details in math term: 
  (1). For any 3D rotation equivalent to a rotation angle stem:[θ] along axis stem:[\vec{v}], there exists a unit quaternion stem:[q=(sin⁡\frac{θ}{2}\vec{v},cos⁡\frac{θ}{2})] to describe this rotation. 
  