This is a mechanism for delimiting the scope of the hypothesis: its sole reason for existence is to establish "B true"; it cannot be used for any other purpose, and in particular, it cannot be used below the introduction.
这是界定假设的范围的机制: 它存在的唯一理由是确立 “B true”；它不能被用做任何其他目的，特别是，它不能被用在这个介入之下。