-- 发布时间：8/6/2006 11:48:00 PM
-- 请教：描述逻辑中2A(A为上标)是什么意思？为什么是2？
最近在看描述逻辑的相关东西，发现经常有2A，2U（A，U是上标）的出现，是什么含义阿？为什么是2呢？属于哪个领域的研究范围呢？ 谢谢！谢谢！
比如：描述逻辑手册第五章中第13页中有如下定义：
The alphabet is † = 2A*{B U {Pdumg}), i.e., the set of pairs whose first component
is a set of atomic concepts, and whose second component is a basic role or the
dummy role Pdum. This corresponds to labeling each node of the tree with a truth
assignment to the atomic concepts, and with the role used to reach the node from
its predecessor.