[Mediawiki-l] The rule for "undo" links?

[Daniel Barrett]

On an article's history page, there are "undo" links next to every single
revision.  However, clickubg some of the undo links displays:

"The edit could not be undone due to conflicting intermediate edits."

Two questions:

1. What is the precise rule for when an undo can/cannot be done?  When
there have been two edits to the same line?  Two edits by two different
authors to the same line?  Etc...?

2. Why does MediaWiki display an undo link next to a revision when the
operation is not possible?  Is it because it's too expensive to check the
"undo status" of every revision when rendering the history page?

Thanks.

DanB