### Has ticket #6480 been fixed?

The ticket is:

```
.subs_expr() method doesn't work for argument of D derivative operator
```

Here is an example (from Sage 6.2, which I'm currently using):

```
var('x,y,f,F,Fx,Fy')
y = function('y',x)
f = function('f',x,y)
d1 = diff(f,x,1)
d1.subs_expr(diff(y,x,1)==F)
F*D[1](f)(x, y(x)) + D[0](f)(x, y(x))
```

So far, all good. But when I try:

```
d1.subs_expr(diff(y,x,1)==F,diff(f,x,1)==Fx,diff(f,y,1)==Fy)
```

I receive the error:

```
TypeError: argument symb must be a symbol
```