Hi!

I am playing around with Julia 1.7.0.

I am using this piece of code:

```
using Symbolics
@variables x
z1(x) = x
z2 = x
Dx = Differential(x)
expand_derivatives(Dx(z1)) #Gives 0
expand_derivatives(Dx(z2)) #Gives 1, which is correct
```

Obviously I am doing something wrong, properly in the anonymous function. What I want to do is symbolically having a function as a function of x, say z(x), then do a differential and end up with a function I can evaluate dz/dx (x) - how would I go about that?

Kind regards