Writing NaN
*Created originally on Bitbucket by [durden (Luke Lee)](https://bitbucket.org/%7Bfce3b3be-76c7-4c30-9909-71d628dc27ba%7D/)* I've written the value `float('nan')` to a spreadsheet, and it doesn't seem like NaN is represented correctly. The cell comes back as 0 after opening the resulting spreadsheet. Should NaN be represented something [like this in Excel](http://stackoverflow.com/questions/15704538/how-can-i-express-nan-values-in-excel/15713107#15713107)?
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