Why there is "不” in “太极不是未有天地之先有个浑成之物，是天地万物之理总名否？” There is this sentence from the opening of the fist chapterof 语类 by Zhu Xi: 


  太极不是未有天地之先有个浑成之物，是天地万物之理总名否？


Baihua version seems to treat the "不是" part as if it ment "是不是" So the sentence would mean something like: 

"Does Tai Ji mean that before Heaven and Earth there was a certain mixture of things and is it [i.e. tai ji] a general name for li of Heaven Earth and all things"

But I do not understand why "不是" should have such meaning? Is it a feature of classical Chinese?  Why shouldn't it mean: 

"Tai Ji is not (不是) a mixture of things that existed before Heaven and Earth, is it [then] a general name for Li of Heaven earth and all things?" 

or soemthing like that???
the text from 朱子語類 卷一 理氣上 has the structure: a is not b, [because] a is c; isn’t it?


  問﹒太極不是未有天地之先﹒有箇渾成之物﹒是天地萬物之理總名﹒否


it would be interpreted as:

[someone] asked (問): “tai chi (太極) is not (不是) [exist] before the universe (未有天地之先), as a (有箇) turbid (渾成) thing (之物),  [tai chi] is (是) the name (名) of the set (總) of principles (理) of everything (萬物) in the universe (天地); isn’t it (否)?

the logic of the second verse is:

thing-1 in the universe has the principle 1 (p1), thing-2 in the universe has the principle 2 (p2), . . . thing-n in the universe has the principle n (pn).

so, the set (mathematics) of these principles is the collection of each principle; a pantheism view.

back to the question, the quoted explanation of baidu is incorrect

have fun :)