Official debt versus real obligations. The official federal debt is some $9.5 trillion, of which about $5.3 trillion is held by the investing public and $4.2 trillion is held in government accounts such as the Social Security and Medicare trust funds. But the official liabilities pale in comparison with the off-book entitlements — obligations to pay current and future Social Security, Medicare and Medicaid benefits. Unfortunately, there is a gigantic gap between the value of future tax collections and the value of future entitlement payments. Economists call this the fiscal gap: the present value difference between the government's projected future...