Hello,

I’m just try to understand the resultant of `diff`

operator. It is a good way to get differentiation with automated way but is it possible to represent the resultant explicitly by UFL representation ?

An example from hyperelasticity:

```
F = Identity(len(u)) + grad(u)
F = variable(F)
.
.
.
```

`#Free Energy Form:`

`psi= (mu/2)*((Ic - 3))- mu*ln(J) + (lmbda/2)*(ln(J))**2`

```
#Exact PK Sress:
S_exact = lambda F: mu * F - mu * inv(F).T + lmbda * ln(det(F)) * inv(F).T
#Alternative PK Stress:
S_alter = diff(psi, F)
```

For simple problems as above, it is simple to take difference between S_exact and S_alter but for complicated energy functions, I cannot be sure about the resultant of `diff`

operator. Is there any way to print the result of S_Alter explicitly as we write S_exact ?

Regards,