How information was acquired may become irrelevant. An obvious case is when
something is confirmed many times. In terms of iterated belief revision, a
specific revision may become irrelevant in presence of others. Simple
repetitions are an example, but not the only case when this happens. Sometimes,
a revision becomes redundant even in presence of none equal, or even no else
implying it. A necessary and sufficient condition for the redundancy of the
first of a sequence of lexicographic revisions is given. The problem is
coNP-complete even with two propositional revisions only. Complexity is the
same in the Horn case but only with an unbounded number of revisions: it
becomes polynomial with two revisions. Lexicographic revisions are not only
relevant by themselves, but also because sequences of them are the most compact
of the common mechanisms used to represent the state of an iterated revision
process. Shortening sequences of lexicographic revisions is shortening the most
compact representations of iterated belief revision states.

信息获取方式可能变得无关紧要，而相同信息的多次确认可能会导致特定修订变得多余，本文给出了一系列词典修订的冗余的必要和充分条件，词典修订不仅在自身上是相关的，还因为它们的序列是表示迭代修订过程状态的最紧凑机制之一。