func (opt *Option) Arg() (string, error)
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:，更多细节参见51吃瓜
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next_url = urljoin(self.base_url, next_a["href"])