Divergence : divergence

### 2.50.4 Divergence : `divergence`

`divergence` takes two arguments : a vector field of dimension n depending on n real variables.
`divergence` returns the divergence of F that is the sum of the derivative of the k-th component with respect to the k-th variable. For example in dimension n=3:

`divergence([A,B,C],[x,y,z])`=∂ A/∂ x+∂ B/∂ y+∂ C/∂ z

Input :

`divergence([x*z,-y```^```2,2*x```^```y],[x,y,z])`

Output :

`z+-2*y`