As knowledge representation tools become more sophisticated, and computer systems increase in power and ubiquity, the prospects of building practical applications based on the representation of large amounts of legislation draw closer. In this paper we reflect on our experience with developing a knowledge representation language for legal rules and an inference engine for this language in the Estrella project, in order to reconsider the principles which should guide the representation of

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... tion. One common demand, based largely on software engineering considerations relating to maintenance, verification and validation, is that representations should be isomorphic to their sources. We explore this notion by representing a fragment of German Family Law using our tools. We show that there are several different ways of representing even this small and simple fragment of law in an isomorphic fashion. Moreover these differences matter, in terms of where the burden of proof is allocated, in terms of the explanations produced, and in terms of the operational procedures that are reflected.