The objective to build a reversible MT system using a series of unification grammars is similar to the objective of the CRITTER system as expressed in , and the work of Zajac in . Instead of using unification grammars CRITTER uses logic grammars; Zajac uses a type system including an inheritance mechanism to define transfer-like rules. In these two approaches less attention is being paid to an exact definition of reversibility; although our work may be compatible with these approaches.
A somewhat different approach is advocated in . In that approach a system is described where an LFG grammar for some source language is augmented with equations that define (part of) the target level representations. A generator derives from this partial description a string according to some LFG grammar of the target language. Instead of a series of three grammars this architecture thus assumes two grammars, one of which both defines the source language and the relation with the target language. The translation relation is not only defined between logical forms but may relate all levels of representation ( c-structure, f-structure, -structure). Although in this approach monolingual grammars may be used in a bidirectional way it is unclear whether the translation equations can be used bidirectionally .
An important problem for the approach advocated here is the problem of logical form equivalence. Shieber  noted that unification grammars usually define a relation between strings and some canonical logical form. Depending on the nature of logical forms that are being used, several representations of a logical form may have the same `meaning'; just as in first order predicate calculus the formulas and are logically equivalent; a unification grammar will not know of these equivalences and, consequently, all equivalences have to be defined separately (if such equivalents are thought of as being translational equivalents); for example in a transfer grammar two rules may be defined to translate into both and if these formulas are thought of as being equivalent. Of course this technique can only be applied if the number of equivalences is finite. It is not possible to define that is equivalent with for any even number of 's.
The approach discussed so far can be extended just as unification grammars for parsing and generation have been extended. Apart from equational constraints it will be useful to add others such as disjunction and negation. Moreover it seems useful to allow some version of universal constraints or some inheritance mechanism to be able to express generalizations and exceptions more easily.