Extract Calculated Fields from an Excel Workbook

I was working with a client last week and the unthinkable happened. The client made a simple editing mistake inside the Calculated Field editing dialogue box and it corrupted the Power Pivot workbook. We could still edit the corrupt workbook, but the Pivot Tables stopped working. Oh No!! Now what? He had written half a dozen new Measures (Calculated Fields) and all that effort now seemed to be lost.