The Government issued bonds to finance its infrastructure projects. The terms of the bond issue provide that interest income of 6% per annum on the bonds shall be tax-exempt if the bonds were to be...

The Government issued bonds to finance its infrastructure projects. The terms of the bond issue provide that interest income of 6% per annum on the bonds shall be tax-exempt if the bonds were to be held up to the end of five years.  Induced by the tax exemption, Mr. Suave purchased bonds worth P10,000,000.00.  Subsequently and before the maturity date (end of five years) of the bonds, Congress passed a law revoking the tax exemption on the interest income on the bonds.  The law passed by Congress is invalid because it violates the:

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