Saving to memory.

1. "in_memory": a dict of variables that are stored somewhere in operating memory, referenced by their names. There you just put that non-serializable thing and access them later just by in_memory["get_that_variable"]
2. "on_disk": a list of names of variables dumped to disk
3. "file_name": a .pkl file name with dump of "on_disk" variables

What happens if the user changes something:

1.  User changes something
2. If len(array) == 10, then we remove the first element of array
3. We create a new dict object
4. We iterate through the list of all the variables the notebook is using (you already have that) and check its types. This is the most general part, as I didn't go deep into the docs. 
   
   If an object is non-serializable, then we add it to "in_memory" (cloning that). If not, then we add its name to "on_disk"
5. We generate new file_name
6. Iterate through "on_disk", dump that
7. Adding this new dict object to the array

What happens if the user presses Ctrl + Z:

1.  We go one position nearer to the beginning of the list. If index == 0, then we're not doing that.
2. We get keys from "in_memory" and restore variables by their names
3. We get names of variables from "on_disk", get contents of dump file, and restore serialized ones

What happens if the user presses Ctrl + Shift + Z:

1.  We go one position nearer to the end of the list. If index == len(array) - 1, then we're not doing that.
2. ...