This is a judgment call.  If a user does the following:

```
> f = grid.open("myfile.txt", "w")
> f.write("hello")
> f2 = grid.open("myfile.txt", "w")
> f2.write("world")
```

afterwards, the way this is currently implemented, there will be just one file in the system named "myfile.txt" that contains just "world".  I think the other option would be, at the end of those lines, for the system two include two "myfile.txt" files, each with a different files_id, one that contained "hello" and one that contained "world".  Thoughts on this?