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In a discussion of powers of Congress in our constitutional system, it makes sense to begin with the enumerated powers of Congress such as the power to coin (or print) money, because these are the "basic powers" that appear in Art. 1 sec 8, of the Constitution itself. One contemporary issue for business (especially those that use optical bill readers), however, involves a different kind of power, because Congress may be changing the face on the $20 bill- from Andrew Jackson to Harriet Tubman. As an offshoot of the power to coin money, this would be an example of Congress' exercise of a/an implied power, the authorization for which can be found in __________________________of the Constitution. There are also important prohibitions on congressional power, such as a provision banning the passage of ex post facto laws, in Art 1 sec 9, of the document.