PSPP-BUG: [bug #24801] No undo function

[Ben Pfaff]

Follow-up Comment #2, bug #24801 (project pspp):

>I can't think of any general and efficient way to undo the effect 
>of an EXECUTE command.

General and efficient?  Me neither.  But it would be pretty easy to undo it
in a general way, by keeping around the old casereader.  The need to preserve
other parts of the dataset (e.g. dictionary) make it slightly nontrivial, but
still it wouldn't be that hard.

The cost, at most, would be doubling the memory and disk requirements (for a
single level of undo).

Do we run EXECUTE after every operation?  We wouldn't necessarily have to do
that; for example, most transformations can be simulated on a per-case basis. 
But that would not be a simple change (even though it would be convenient for
users).

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