But even a modest official devaluation might have encouraged ordinary Russians to panic too, producing a full-blown run on the currency.

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The dollar jumped to a high of Y94.42 after Lael Brainard, the top US Treasury official for international affairs, restated US support for monetary stimulus in Japan but warned against competitive devaluation.

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Francisco Lopes, briefly appointed governor of the central bank at the time of Brazil 's devaluation in January, refused to testify under oath to a Senate inquiry into leaks of official information.

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Although the devaluation of sterling we've seen in the last few years - caused by a near-zero official interest rate and the massive money creation of quantitative easing - has probably been healthy, especially for exporters, more extreme degradation of the currency would bring risks.

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