### Calculus with formal functions: substitution?

If I enter

```
var('f,x,y')
y = function('y',x)
f = function('f',x,y)
```

then

```
f.diff(x)
```

returns

```
D[1](f)(x, y(x))*D[0](y)(x) + D[0](f)(x, y(x))
```

In order to perform more computations later on, I want to rewrite this, using substitutions, so it looks something like

```
Fy*yx + Fx
```

But I've got no idea how to do this. In Maxima I'd just use `'diff(f,x)`

as the expression to substitute for, but even trying to use `dummy_diff`

doesn't seem to work:

```
from sage.calculus.calculus import dummy_diff
dummy_diff(f,x)
```

produces the same result as `f.diff(x)`

above.

Does anybody know how I can use substitutions to express the results of a formal differentiation with other symbols?