title: Sympy symbolic differential
body: <p>So, I have a equation like this <a href="https://i.stack.imgur.com/etAhP.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/etAhP.gif" alt="enter image description here"></a>. Now if I operate grad function on this it becomes like this</p>
  
  <p><a href="https://i.stack.imgur.com/CFh0K.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CFh0K.gif" alt="enter image description here"></a></p>
  
  <p>How do I do this on sympy? Sympy uses a reference frame but I didn't find any other than x,y,z, And I need multiple custom variable like this. Is it possible to do it in sympy?</p>