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Like WikiMedia, FlexWiki should implement a system where people can explain why they made an edit. The input field would appear near the submit button. The responses would appear on the RecentChanges page. The field would not be a mandatory field.
I agree this would be useful, especially for the way we use Flexwiki as a collaboration tool across a dispersed team.
I would suggest that ideally the comment optional/mandatory could be set as an attribute of the page.