### Substitute differential operators in an expression.

Let's say I am taking derivatives of an expression involving unknow function.

```
var('x,a,b');
f=(x^(a+b)).function(x,a,b);
h=function('h',nargs=1)(x);
g=h(f(x,a,b));
dg=diff(g,x);
dg
```

This gives the output

```
(a + b)*x^(a + b - 1)*D[0](h)(x^(a + b))
```

How do I replace `D[0](h)(x^(a + b))`

with something like `direvative_of_h(x^(a + b))`

?
In this simple case, I can just do this manually using `.operands`

, but if I have a rather complicated equation, how I can I do it?