### symbolic functions and expressions

Suppose I have a huge expression involving symbolic variables and symbolic functions. Is there a way to get the symbolic functions involved in my expression? Something like `.variables()`

.

For example:

```
f(x) = function('f')(x)
h(x) = function('h')(x)
Y(l,m,x,y) = spherical_harmonic(l,m,x,y)
A = f(x)*x^2 + x^diff(h(x),x) + h(x)*Y(l,m,x,y)
```

`A.desired_function()`

should return `(f(x), h(x), diff(h(x),x), Y(l,m,x,y))`

. Or, at least `(f(x), h(x), diff(h(x),x))`

. I was not able to find such function nor figure out a way build up one.