Two level credibility-limited revision is a non-prioritized revision operation. When revising by a two level credibility-limited revision, two levels of credibility and one level of incredibility are considered. When revising by a sentence at the highest level of credibility, the operator behaves as a standard revision, if the sentence is at the second level of credibility, then the outcome of the revision process coincides with a standard contraction by the negation of that sentence. If the sentence is not credible, then the original belief set remains unchanged. In this paper, we propose a construction for two level credibility-limited revision operators based on Grove's systems of spheres and present an axiomatic characterization for these operators.

翻译：两级可信度限制修正是一种非优先修正操作。在进行两级可信度限制修正时，需考虑两个可信度层级和一个不可信层级。当以最高可信度层级的语句进行修正时，该操作表现为标准修正；若语句处于第二可信层级，则修正过程的结果等同于对该语句的否定进行标准收缩。若语句不可信，则原始信念集保持不变。本文提出了一种基于格罗夫球系系统的两级可信度限制修正算子构造方法，并给出了这些算子的公理化刻画。