As an incident to the exercise of any of the powers expressly given to the federal government, Congress may provide for the punishment as crimes of acts calculated to interfere with such exercise of its powers. Thus under the power to establish post offices and post roads, Congress has provided for the punishment of a variety of acts calculated to interfere with the safety or efficiency of the postal service; and under the power to lay and collect taxes, it has provided for the punishment of various acts of fraud or evasion with reference to the payment of import or excise duties. The scope of the federal power to punish crimes against the operations of the government is therefore very extensive, and a general enumeration of the classes of offences defined by Congress in the exercise of its implied powers would be impracticable.