I write all of my prose in plain text. The format I use is markdown/multimarkdown for all of the reasons markdown became a thing; it's easy to read as-is and it's easy to parse and render into richer formats like HTML. As an aside, I may switch to ASCIIDOC after reading Scott Chacon's excellent piece on how he used it to revolutionize his technical writing. But that's another post for the future!
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