Controlled Legal German (CLG) is a controlled natural language being developed for the representation of legal norms contained in Swiss statutes and regulations. This paper discusses the main design requirements CLG faces and the strategies it applies to meet them. CLG aims at providing representations for legal norms that are both formal and can be easily understood and veri ed by legal experts. It must combine an unambiguous semantics based on FOL and deontic concepts with close syntactic proximity to conventional legal language.
The development of knowledge-based legal information systems has been at the
forefront of research in arti cial intelligence and law for the last couple of decades
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We are in the process of developing Controlled Legal German (CLG), a logicbased controlled natural language designed for the representation of norms codi ed in Swiss statutes and regulations. CLG aims at providing representations that are formal and can be easily understood and veri ed by legal experts. In this paper, we will rst give a brief overview of the state of development of the project (section 2) and then focus on the speci c design requirements that arise from our target application and the strategies we pursue to meet them in CLG (section 3). The paper concludes with a brief discussion of some problems and limitations and an outlook to future research (section 4). 2
The project is currently in its rst phase, in which we specify the syntax and semantics of the controlled language: we de ne what constructions CLG sentences can consist of and what formal logical representations they map onto. In a second phase, we plan to devise the computational tools needed to automatise the mappings involved: a parser that translates CLG statements into the formal logical representations speci ed in the de nition of the language and a converter that makes the interpretations assumed in CLG explicit by generating respective paraphrases (cf. section 3).
CLG restricts the syntax and semantics of Swiss legal language to prevent
instances of ambiguity arising from constructions that either have more than
one syntactic analysis (syntactic ambiguity) or whose syntactic analysis can be
mapped onto more than one non-equivalent logical structure (semantic
ambiguity). Construction rules prohibit the use of certain ambiguous constuctions;
interpretation rules assign a default interpretation to others and suggest
paraphrases for excluded readings. Like in Attempto Controlled English [
CLG's unambiguous formal semantics is based on rst-order predicate logic augmented with defeasible rules (A ; B) and the deontic concepts of obligation (O), permission (P) and prohibition (:P). We have chosen a formal underpinning that is expressive and \deep" enough to capture the essential content of individual norms { who must or may do what, how and under what circumstances? { and yet generic enough to be easily converted into other formats of formal representation.
The current version of the language, CLG 1.0, provides the basic syntactic and semantic inventory to express simple norms (obligations, permissions, prohibitions; including norms stating duties and responsibilities). It comprises roughly two dozen construction and interpretation rules that deal with phenomena such as attachment ambiguities (PPs, relative clauses), plural ambiguities (distributive/collective readings), scope ambiguities, lexical ambiguities (function words), referential ambiguities (pronouns, relational nouns) and functional ambiguities due to the relatively free German word order. We will exemplify some of these rules in the next section. The language does not yet include elements of temporal and intensional logic; consequently, it does not yet permit the use of tenses other than present tense and subordinate clauses other than conditional and relative clauses. We plan to add these concepts in CLG versions 2.x and 3.x respectively.
The design of CLG has been guided by the two operations it must support: (a) the translation, by hand, of legislative texts into CLG, carried out by knowledge engineers; (b) the veri cation of the CLG representation by legal experts. The translation of legislative texts into CLG is the easier the closer CLG resembles the language used therein. Moreover, proximity to conventional legal language will make lawyers feel more at ease with CLG transcriptions of statutes and regulations and thus facilitate veri cation by domain experts. On the other hand, the correctness of CLG transcriptions can only be veri ed by domain experts if default interpretations are made explicit.
CLG thus needs to accommodate two somewhat converse requirements: (a)
proximitiy to conventional legal language and (b) maximal explicitness. It does
so by two means. On the one hand, CLG exploits the conventions and frequency
distributions of ordinary legal language. Some constructions are ambiguous in
full natural language but not in the language of Swiss statutes and regulations,
and for some ambiguous constructions, one interpretation is used more frequently
in legislative texts than the other. Wherever possible, such conventions and
frequency distributions are re ected in CLG's construction and interpretation rules
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As an example, suppose a knowledge engineer needs to formalise the following norm: (1) Die Bundesversammlung erlasst rechtsetzende Bestimmungen in der Form des Bundesgesetzes [...]. (Art. 163 Abs. 1 Swiss Federal Constitution) `The Federal Assembly enacts legislative provisions in the form of the federal statute.' It is not entirely clear whether this is (a) a norm about the responsibilities of the Federal Assembly or (b) a norm about the enactment of legislative provisions by the Federal Assembly. In CLG, the two interpretations can be expressed as shown in (2) and (3) respectively. For each reading, we give a CLG representation that is close to conventional legal language (C), an explicit paraphrase equivalent to it (E), and the underlying formal semantics1 (F). (2) C: Die Bundesversammlung erlasst rechtsetzende Bestimmungen in der
Form des Bundesgesetzes.2 1 For the convenience of the reader, predicate names have been rendered in English. 2 Note that this CLG representation is positively identical to the original text; the reading it stands for is thus particularly easy to extract.
`The Federal Assembly enacts legislative provisions in the form of the federal statute.' E: Es ist zwingend, dass die Bundesversammlung eine oder mehrere rechtsetzende Bestimmungen [in der Form des Bundesgesetzes erlasst]. `It is obligatory that the Federal Assembly [enacts in the form of the federal statute] one or several legislative provisions.' F: 9!x : federal assembly (x ) ^
O O 9ey : legislative provision(y ) ^ enacts(e; x ; y ) ^
O 9ey : in form of federal statute(e)3 (3) C: Der Erlass von rechtsetzenden Bestimmungen durch die Bundesversammlung erfolgt in der Form des Bundesgesetzes. `The enactment of legislative provisions by the Federal Assembly is e ected in the form of the federal statute.' E: Es ist zwingend, dass jeder Erlass einer rechtsetzenden Bestimmung durch die Bundesversammlung in der Form des Bundesgesetzes erfolgt. `It is obligatory that every enactment of a legislative provision by the Federal Assembly is e ected in the form of the federal statute.' F: 9!x : federal assembly (x ) ^
O O 8ey : enacts(e; x ; y ) ^ legislative provision(y ) !
O 8ey : in form of federal statute(e) The paraphrases make the following CLG default interpretations underlying the conventional CLG representations explicit: (a) norms without modal verb are considered obligations (by preceding the sentence with the phrase es ist zwingend, dass), (b) PPs attach to the verb (by inserting brackets), (c) inde nite NPs in non-vorfeld position are interpreted as existentially quanti ed rechtsetzende Bestimmungen (by using determiners such as eine oder mehrere), and (d) nominalised verbs in vorfeld position (here der Erlass ) are interpreted as universally quanti ed (by adding the determiner jeder ). 4
The fact that legislative language is highly conventionalised facilitates the task of
designing a resembling controlled natural language to some degree [
The provision of explicit paraphrases often poses further problems: it is simply not possible to nd explicit paraphrases for all constructions to which CLG applies an interpretation rule. We have tried to partially overcome this problem 3 Adverbial phrases are not yet analysed in CLG 1.0 but treated as atomic conditions. by admitting non-linguistic elements in these paraphrases. An example are the brackets we use in (2E) to indicate the attachment of constituents.
The foremost limitation of our project lies in the fact that the eld of arti cial intelligence and law has not yet arrived at developing a logical formalism capable of adequately representing the full content of statutory law. For the moment, we have therefore chosen a formal underpinning for CLG that is expressive and \deep" enough to capture what we deem the essential content of individual norms { who must or may do what, how and under what circumstances? { and yet generic enough to be easily converted into other formats of formal representation. We believe that with such a generic representation, chances are best that a future integration into arti cial intelligence and law systems that model legal reasoning will be possible. Such systems will, of course, also have to take into account information that is not contained in the actual norms that we want to represent in CLG. Examples are the precedence of more speci c norms over more general norms or the precedence of constitutional provisions over provisions stated in statutes or regulations.
Finally, the notorious underspeci city of normative stentences continues to pose a problem for their translation into a controlled natural language: many relations within sentences and between sentences (bridging references, ellipses, rule-exception relations) are not made explicit in normative texts but are left to be inferred from the context. In a controlled natural language, such relations have to be stated explicitly. Providing formalisms for doing so will be one of the largest task to be tackled by our future research.