Several tax systems impose a regular income tax and, where higher, an alternative tax. The U.S. imposes an alternative minimum tax based on an alternative measure of taxable income. Mexico imposes an IETU based on an alternative measure of taxable income. Italy imposes an alternative tax based on assets. In each case, where the alternative tax is higher than the regular tax, a credit is allowed against future regular tax for the excess. The credit is usually limited in a manner that prevents circularity in the calculation.