# How to take partial derivative of abstract/unknown function?

I have a function `f(x)`

that I do not want to be expanded but wish to evaluate the partial derivative of `g(x,f(x),...)`

with respect to `x`

, I can not figure out how to declare the function and indicate it is a function of x without an explicit definition.

Code example:

```
var("x")
f = ??? # how do I define this?
g = x * f(x)
g.diff(x)
```

Expected output:

```
x * f'(x) + f(x)
```