A hypothesis is falsifiable if there exists a logically possible observation statement or set of observation statements that are inconsistent with it, that is which, if established as true, would falsify the hypothesis.
其中， that is which，that的语法功能，which的语法功能。
这是多重复合句
【第一重】
主句：A hypothesis is falsifiable 假设是可以被检验的
条件从句：if there exists a logically possible observation statement or set of observation statements that areinconsistent with it, that is which, if established as true, would falsify the hypothesis.
【第二重（嵌套在条件从句中）】
主句：there exists a logically possible observationstatement or set of observation statements （如果）在理论上存在一个或一组观察数据陈述
定语从句：that are inconsistent with it, that is which, if established as true, would falsify the hypothesis. <that 是关系代词，指代前文的“一个或一组观察数据陈述>
【第三重（嵌套在定语从句中）】
插入性条件从句：if established as true 如果它们被确定是真实的 <= it or they are establishedas true >
主句：that are inconsistent with it 这种观察数据陈述与假设前后矛盾
同位语：that is which would falsify the hypothesis 也就是说这种观察数据陈述歪曲了假设 <that is 相当于 that is to say、 namely、in other words >.
请问老师，在第三重中，which 如何理解其语法功能？
来自：求助得到的回答