### derivatives of variable order

Using `diff/derivative`

it does not seem easily possible to specify derivatives of variable order, e.g., to define a function `P(m,n)`

that applies the `diff`

`m+n`

times to a polynomial:

```
sage: x=var('x')
sage: P(m,n)=derivative((1-x^2)^n,x,m+n)
...
TypeError: argument symb must be a symbol
sage: R.<x> = PolynomialRing(QQ, 'x')
sage: p=(1-x^2)^11
sage: P(m,n)=derivative(p,x,m+n)
...
ValueError: Cannot differentiate with respect to m + n
```

This is obviously because `diff`

supports such syntactic sugar like `diff(p,x,x,x)`

. So how to differentiate to a variable order?