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As used in this chapter in respect of the tax imposed by this chapter the term 'deficiency' means—\n(1) The amount by which the tax imposed by this chapter exceeds the amount shown as the tax by the donor upon his return; but the amount so shown on the return shall first be increased by the amounts previously assessed (or collected without assessment) as a deficiency, and decreased by the amounts previously abated, refunded, or otherwise repaid in respect of such tax; or\n(2) If no amount is shown as the tax by the donor upon his return, or if no return is made by the donor, then the amount by which the tax exceeds the amount previously assessed (or collected without assessment) as a deficiency; but such amounts previously assessed, shall first be decreased by the amounts previously abated, refunded or otherwise repaid in respect of such tax.