>You could build some functions to manipulate fragments from XML, then build some higher-level functions from those, etc., then traverse your data (whatever it is) and use the functions you've built to emit XML in a clean and _precise_ way. I think Haskell already has such a library -- perhaps someone's ported it to F#?

Are you talking about Parser combinators? haskell [http://www.haskell.org/haskellwiki/Parsec] and F#'s FParsec [http://www.quanttec.com/fparsec/]?