This reduces the burden of child-rearing on the parents and cuts the implicit cost of children.

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Yet, even this underestimates the implicit cost of breach.

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Right at the opening, there was a spike in the implicit interest cost to Italy of borrowing for two years (the yield on two-year bonds) to a euro-era record of around 8% - and then it fell back to 7.5% or so, which is where it was on Friday (which is still high).

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Impose a fee on banks whose size exceeds a certain percentage of GDP to cover the cost they would impose on taxpayers in a bailout, thus eliminating the implicit subsidy of their too-big-to-fail status.

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And the cost to the supplier of immediate cash payment by the bank, in respect of the implicit interest rate or discount on the bill, ought to be tiny - because the bank is in effect lending to the safe big company, which ultimately honours the bill, not to the riskier supplier.

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