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...

INTRODUCTION Corpus linguistics is a branch of linguistics in which the scholars analyse large collections of electronic documents. For English there are many such corpora. The earliest and best known is the Brown Corpus [1]. The Linguistic Data Consortium at the University of Pennsylvania is collecting several hundred million words of English. The Text Encoding Initiative (TEI) [2] is specifying SGML Document Type Definition standards for corpus linguistics, which have already been put to use in the 100 million word British National Corpus (BNC) [3]. The International Corpus of English (ICE) [4], of which one of us (Meyer) leads the American effort, is collecting 1 million words of written and spoken English from each of 15 countries. To be useful for electronic analysis, documents once transcribed into electronic form must have some conventional markup inserted to mark the boundaries of whatever linguistic constructs are to be studied. Some of this markup, such as that impl