Sentence:  That the tax would take effect after some of the payments were made also raises issues. Says former Attorney General Richard Thornburg, also in an e-mail: "Such legislation could well run afoul of constitutional restrictions on bills of attainder, <WSD>ex post facto</WSD> laws, laws impairing the obligations of contract, unauthorized takings of property and the like." 
Meaning of the word marked with <WSD> tags:  Formulated or enacted after some event, and then retroactively applied to it.