No ex post facto law shall be passed.

Unless you're Chuck Grassley and you chair the Senate Finance committee. Apparently he wants to pass legislation early next year closing various tax loopholes relating to vehicle donation and building restoration, and he wants them to be retroactively applied.

• Sen. Chuck Grassley, chairman of the Committee on Finance, and Sen.
  Max Baucus, ranking member, today said they plan to put forth legislation early next year that would increase and create additional fines and penalties on promoters, taxpayers and appraisers who participate, aid or assist in the donation of facade easements found to be significantly overvalued. The senators plan to make the additional fines and penalties effective today.