def robotsrulesetOfDomain(self, domain):
		'''检查domain是否在数据库中,
			否 --> (False, None)
			是 --> (True, 数据库中存储的robots.txt内容)
		'''
		exist = False
		cur = self.conn.cursor()
		# 是否存在
		cur.execute("SELECT 1 FROM Domains WHERE domain=?", (domain,))
		if cur.fetchone() :
			exist = True
		# 存在的话,结果是什么
		cur.execute("SELECT rules FROM Domains,Rulesets "
			"WHERE domain=? AND Domains.did=Rulesets.did"
			,(domain,) )
		ruleset = cur.fetchone()
		return (exist, ruleset)
def __init__(self, head):
        self.head = head
        self.lsize = 0
        while head.next:
            head = head.next
            self.lsize += 1

        self.m1_idx = None
        self.m2_idx = None
        if self.lsize > self._largesize:
            self.m1_idx = self.lsize / 3   # start from 1/3
            self.m1 = self._getN(self.m1_idx)
            self.m2_idx = self.m1_idx * 2  # start from 2/3
            self.m2 = self._getN(self.m2_idx)