this paper, we propose a general framework for implementing undo in collaborative systems. The framework allows users to individually reverse their own changes, taking into account the possibility of conflicts between different users' operations that may prevent an undo. The proposed framework has been incorporated into DistEdit, a toolkit for building group text-editors. Based on our experience with DistEdit's undo facilities, we discuss several issues that need to be taken into account in using the framework, in order to ensure that a reasonable undo behavior is provided to users. We show that the framework is also applicable to single-user systems, since the operations to undo can be selected not just on the basis of who performed them, but by any appropriate criterion, such as the document region in which the operations occurred or the time interval in which the operations were carried out. Categories and Subject Descriptors: D.2.2 [Software Engineering]: Tools and Techniques -- User Interfaces; H.1.2 [Models and Principles]: User/Machine Systems -- Human Factors; H.2.2 [Database

The ability to undo operations is a standard feature in most single-user interactive applications. However, most current collaborative applications that allow several users to work simultaneously on a shared document lack undo capabilities; those which provide undo generally provide only a global undo, in which the last change made by anyone to a document is undone, rather than allowing users to individually reverse their own changes. In this paper, we propose a general framework for undoing actions in collaborative systems. The framework takes into account the possibility of conflicts between different users' actions that may prevent a normal undo. The framework also allows selection of actions to undo based on who performed them, where they occurred, or any other appropriate criterion. The proposed framework have been incorporated in DistEdit, a toolkit for building text group editors. Based on our experience, we discuss methods for maintaining the undo information in a groupware env...