The dynamic description logic DDL provides a kind of action theories based on description logics (DLs). Compared with another important DL-based action formalism constructed by Baader et.al., a shortcoming of DDL is the absence of occlusions and conditional postconditions in the description of atomic actions. In this paper, we extend atomic action definitions of DDL to overcome this limitation. Firstly, we introduce extended atomic action definitions in which the occlusions and conditional

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... conditions are incorporated. Secondly, for each atomic action specified by an extended atomic action definition, a function named Expand is introduced to transform it into a choice action which is composed of atomic actions defined by ordinary atomic action definitions. Finally, based on the Expand function, the satisfiabilitychecking algorithm of DDL is extended to support occlusions and conditional post-conditions.