Frank cast about in his mind for some mitigating information that would make the ladies feel better.
Some of the feeling of bitter hatred the women bore Scarlett for her share in the tragedy was mitigated by the knowledge that her husband was dead and she knew it and could not admit it and have the poor comfort of claiming his body.
There are no more uses of "mitigate" in the book.
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Don’t judge her so harshly until you consider the mitigating circumstances.
The new factory is building privacy walls to mitigate its impact on neighbors.