### Making a dictionary of matrices, and save the session

Some time ago I made a dictionary of matrices with your help.

Now I like to save the session by `save_session()`

but somehow I fail. The terminal goes as follows

```
sage: A=matrix(GF(2),[[1,0],[0,1]])
sage: A.set_immutable()
sage: dictA={A:1}
sage: save_session(verbose=true)
Saving A
Not saving dictA: mutable matrices are unhashable
```

On the other hand

```
sage: A=matrix([[1,0],[0,1]])
sage: A.set_immutable()
sage: dictA={A:1}
sage: save_session(verbose=true)
Saving A
Saving dictA
```

I have no idea why specifying `GF(2)`

fails `save_session()`

. Does anyone know a fix?