6 Dec
2004
6 Dec
'04
9 a.m.
Well, if you consider large word-processors like word keep a complete undo history, I would keep a document as a list of changes:
data Document = Insert Location String | Delete Location Location ...
Better to store it as a pair of the most recent state, and a list of *reversed* changes.
--KW 8-)
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Keith Wansbrough