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As President Obama packs his bags for Asia, he'd better throw in plenty of antacids, iron supplements, and earplugs. He'll need all three if he is to resist mounting pressure at home to throw a public fit at the Chinese for holding their currency, the renminbi (RMB), or the yuan, artificially low, to the obvious detriment of other economies, not least America's.  Coming from critics as prestigious and generally pro-Obama as Nobel economics laureate Paul Krugman, writing in his Oct. 22 New York Times column, that pressure is substantively valid, politically compelling -- and best ignored.First, for the valid part: By holding the yuan down, Chinacheapens the prices of its exports and effectively raises the prices of its imports,  thus disadvantaging the economies of other countries,...