Monolingual and bilingual knowledge is represented in a declarative way. Declarativity implies that the grammar writer does not have to worry about the actual processing of the linguistic knowledge he/she encodes, but only worries about the logical meaning of a grammar. Declarativeness has been argued for from a computational point of view because it implies that different compilers and interpreters may be applicable to the very same program. This has led to the bidirectional use of programs written in declarative grammar formalisms such as PATR and DCG . Some recent developments are reported in .
We make a distinction between symmetric and reversible. We call a translation relation reversible if is symmetric and computable. Symmetry of the `possible translation' relation has been argued for above. A relation is called computable iff for a given the set can be enumerated by some terminating procedure. We will return to this matter in section 2.1.
Reversible systems are preferable to nonreversible ones. The arguments in favour of using bidirectional grammars in NLP, such as those given in  carry over to translation. Furthermore Isabelle  claims that reversible MT systems are to be preferred to others because in reversible MT systems a better understanding of the translation relation is achieved; such systems will eventually exhibit better practical performance. Monolingual grammars that are used only for analysis will often allow constructions that are in fact ungrammatical. As an example consider English auxiliaries. Suppose that the English auxiliaries are analyzed as verbs that take an obligatory -complement. Moreover each auxiliary may restrict the (participle, infinite) of this complement. This allows the analysis of sentences such as `John will have been kissing mary'. However, the possible order of English auxiliaries (eg. `have' should precede `be') is not accounted for and the analysis sketched above will for example allow sentences such as `John will be having kissed Mary'. The strictness coming with a bidirectional grammar will be useful for analysis too, because strictness usually implies less local and global ambiguities.