// Use it directly
12月21日，有着560多年历史的先农坛庆成宫首次面向社会公众开放。
。WPS官方版本下载对此有专业解读
I learned that for 4-SAT, if clause to variable ratio is more than 10, the generated problems become difficult to solve, and the likelihood of formula to be SAT or UNSAT is close to 50%. So I generated 3 types of formulas:
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