reading worksheet and preserving conditional formatting

I am trying to read an excel worksheet with openpyxl. I think I am losing the conditional formatting information in the sheet when I read it like so:

xl = openpyxl.load_workbook(filename)

When I read all cells in the file and save it. I get a spreadsheet in which none of the conditional formatting is implement.

I can find many ways of adding conditional formatting to a spreadsheet at http://ift.tt/187c3Dv

But I cannot find a way to read conditional formatting information in an existing worksheet.