Bats in Churches
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That is not only a sadness and a loss, but becoming an aged society is a cultural and economic threat. Older people, by and large, are not the innovators or new thinkers. An ageing society risks declining in optimism, creativity and, above all, risk-taking: a top-heavy preponderance of older people makes for a conservative and fearful electorate. We are there already – and it’s getting worse.，更多细节参见同城约会
For UNSAT problems with 10 variables and 200 clauses, it always claimed that the formula is SAT and made up assignments (See this example).