Moreover, these aspects are very important if an NLS should be portable across different domains. In particular, if the same system is to be used as a front-end component in the case of understanding and as a back-end component in the case of generation (systems like HAM-ANS [Hoeppner et al. 1983], XTRA [Allgayer et al. 1989], DISCO [Uszkoreit et al. 1994] are developed exactly to serve in that sense) reversibility ensures that a distinct subset of the linguistic knowledge of that system is reusable between different tasks. This is of great importance for the flexibility of a system. For example, one of the disadvantages of current generation systems is that they view the structure of linguistic knowledge only statically. If alternatives exist for a particular linguistic expression, decision points are evaluated to determine the appropriate actual utterance. It is necessary to specify corresponding decision points for all possible utterances; otherwise the choice must be performed randomly (the determination of the appropriate set of decision points is one of the sources of complexity in existing generation systems). The flexibility of such systems depends directly on the flexibility that is brought into the system via the decision points that are specified by hand during the development of a generation system (i.e. the flexibility is restricted).
When using a reversible system, structures resulting from the parsing task can be used directly during generation. This reduces the number of decision points or parameters which have to be specified during the development of an NLS which leads to more flexibility: not all necessary parameters need to be specified in the input of a generator because decision points can also be set dynamically at run-time. Consider, for example, the problem of possible ambiguity of a produced utterance. In many situations of communication a speaker need not worry about the possible ambiguity of what she is saying because she can assume that the hearer will be able to disambiguate the utterance by means of contextual information or would otherwise ask for clarification. But in some situations it is necessary to avoid the risk of generating ambiguous utterances that could lead to misunderstanding by the hearer, e.g., during the process of writing text, where no interaction is possible, or when utterances refer to actions that have to be performed directly, or in some specific dialog situations (e.g. having an interview with a company). When a reversible grammar is in use it is possible to directly compare the grammatical structures obtained during parsing and generation of an utterance. This helps to identify the relevant sources of ambiguity.