This example differs from the example in DiaLaw. Here the content of a Court decision was taken over, whereas in DiaLaw the arguments of the court were adduced but defeated by a counter-argument (the municipality was negligent). We do not say that one outcome is better than the other. Both outcomes are defensible, and just show another aspect of argumentation. It would have led to far astray to show an example in which the decision of the Court was criticised by arguments of expert lawyers (like the already mentioned tort specialist professor Van Maanen), but these discussions must be possible if teaching systems are built, e.g. based on either DiaLaw or CumulA or on a combination.
4.3 Opportunities of the Graphical Approach
The implementation of CumulA, illustrated above, can be used in legal education for training several types of argumentation skills:
Students are forced to make arguments in an explicit reason-conclusion structure. For instance, they become aware of the fact that the same statement can be used as the reason in an argument and as the conclusion of an argument.
Students see that assumptions are needed to justify conclusions and how justified reasons make their conclusion justified.
Students can practice the different ways of using legal rules (as warrants behind reason/conclusion-connections) and legal cases (as support for conclusions) in arguments.
Students experience how counterarguments can be used to defeat arguments, that were previously undefeated. They undergo the successive changes of status. The relation of counterarguments and argument-structure is also clarified.
Students get a feeling for the role of process in argumentation by the gradual construction of arguments and counterarguments, and by the occurrence of status changes.
In training these skills the graphical approach to the mediation of legal argument can be beneficial, e.g.:
The graphical lay-out gives direct insight in the reason-conclusion structure in a line of argumentation.
The graphical lay-out directly shows which arguments are counterarguments to other arguments.
The changes of statuses (e.g., from undefeated to defeated) are directly noticeable by graphical changes.
The graphical approach is not appropriate in all respects. For instance, it partly requires an unusual attitude towards argumentation. People do not necessarily think in graphical terms of explicit reason/conclusion-structures. The use and presentation of counteraguments is even less familiar. A more fundamental issue is the limitation of the graphical interface. A long line of argument can easily go 'off-screen' and result in a complex and hard to understand structure of statements, reason-arrows, and counterargument-structures.6 The ArguMed-system [Verheij 1998b] has been developed as a successor to the Argue!-system in an attempt to enhance the familiarity of the interface and the transparency of the underlying argumentation theory.7
5. Towards a Hybrid Approach
Verbally presenting arguments has a long tradition. The old Greeks already paid attention to argumentation, understandably in verbal style only, while focussing on the core element of current mediating systems: dialectic. One of the first graphical approaches, and yet still widely used in AI & Law, are the argument schemata by Toulmin .
We have suggested how both the verbal and the graphical approach to the mediation of legal argument can be useful as tools in teaching legal argument. The verbal approach fits in nicely with legal practice. As a result, students can practice skills they need in their professional career. They are trained in choosing arguments with the right meaning and rhetorical power.
The graphical approach has the advantage that it can provide a clear overview of a line of argumentation at a glance. One easily 'gets the picture'. The graphical approach also forces to think of arguments in a new way, namely in terms of pictures.
An additional tool of DiaLaw that has been formally defined, but is not implemented yet is that the dialog history can be depicted by means of trees [Lodder, 1998]. By way of the dialog trees a good insight into the layered structure of the dialogs is provided. Implementing these trees could be a first step towards combining the verbal and graphical approaches.
We recommend that both the verbal and the graphical approach are combined in one system, in order to profit from the best of each. E.g., the system could provide means to switch between different presentations. In order to encourage students to use the system, a game-like element is essential. In a dialog game they can try to win by beating their opponent, e.g., by drawing a convincing argument.
We think that integrating systems such as Argue! and DiaLaw could lead to a good result. Obviously, a hybrid system does not have to be based on an integration of the systems described in this paper, and we hope that others take up the thread by realising a genuine hybrid system for the mediation of legal argument. The next step would then be to use and test the system in an educational environment.
In this paper opportunities of computer-mediated legal argument in education are suggested. Two approaches to the presentation of arguments, the verbal and the graphical, have been discussed. The DiaLaw system and the Argue! system have been described as examples. Although both approaches have their specific merits, we think that a combination of the two would be most promising in an educational environment. We recommend that future research on automated tools for teaching legal arguments focuses on developing systems in which arguments are presented both graphically and verbally. By the development of attractive systems, e.g., with a game element, students can be encouraged to train their argumentation skills. Since training in these skills does often not receive much attention in the overloaded curricula of legal education, computer-mediated legal argument could become a valuable addition to argumentation courses in legal education.
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1 See, e.g., the work of Freeman and Farley , Gordon , Hage [1996, 1997], Lodder and Herczog , Loui and Norman , Prakken , Prakken and Sartor , Verheij , and Yoshino . Verheij, Hage and Lodder  give an overview.
2 Supposedly, since most effort has been spent on theoretical research.
3 An example of a mediating system used for educational purposes taking the verbal approach is [Moore, 1993].
4 The HELIC-systems [Nitta & Shisabaki, 1997] aim to model argumentation between a human-player and a computer-player.
5 An example of a mediating system used for educational purposes taking the graphical approach is Statutor [Routen 1991; Centinia et al., 1995].
6 Loui et al.  provide a partial solution to this problem of ‘pointer spaghetti’. Instead of using arrows to connect reasons and conclusions, they use boxes inside other boxes.
7 See Verheij's web site on automated argument assistance .