A unification grammar defines the transfer relation between logical forms of two languages. Like in generation, the `input attribute' is a logical form. Instead of strings, logical forms of the target language are generated.
For example the input to transfer may be a feature structure such as figure 5 for a sentence such as `The army opens fire at the civilians' where contains the English logical form and the Spanish logical form .
The bilingual grammar will apply its rules, testing after each application whether the value of the attribute subsumes the input feature structure. The value of the attribute will gradually be instantiated. At the end of the process, the system will test whether the input feature structure subsumes the value of the attribute . If this is the case, then the value of will be considered the output of transfer. An example of a simple rule in PATR notation is given in Figure 6. The integers are names of feature structures, where will normally be used to represent the mother node and represent the daughter nodes. Application of this rule to the feature structure of Figure 4 results in the three instantiations in Figure 7.
An example of the rule for the first daughter will be a lexical entry and may look as in Figure 8. The simple English expression `army' has to be translated as a complex expression in Spanish: `fuerza militar'. The rule will look as in Figure 9 where it is assumed that the construction is analysed in Spanish as an ordinary noun-adjective construction, and where the logical form of the adjective takes the logical form of the noun as its argument. The translation for `civilian' is defined in a similar rule (although the translation of `number' is different). Note that this example of complex transfer is similar to the famous `schimmel - white horse' cases. As a result of the rule applications the value of the attribute in Figure 5 will get instantiated to the feature structure in Figure 10, from which the generator generates the string `La fuerza militar rompio el fuego a la poblacion civil'.
For some transfer equivalences a rule such as in Figure 6 will be too simplistic. The rule in Figure 11 is used to translate a logical form such as into the Dutch equivalent , as in for example `Minister Kok likes the reforms' vs. `De hervormingen bevallen minister Kok'. Note that the attribute contains the Dutch logical form.
The logical forms that are encoded in MiMo2 are more complex than in the foregoing examples. For example attributes for and may be present to represent information about voice, tense and aspect. The example in Figure 11 shows that it is sometimes necessary to alter the value of `voice'. The Dutch logical form with is related to with . This rule may look as in Figure 12.
This approach is not entirely without problems. It seems that some redundancies might be inevitable with the architecture proposed in section 2. A monolingual grammar defines a relation between strings and logical forms. It thus defines possible logical forms as well. Similarly, a bilingual unification grammar will have to define possible logical forms. The notion `possible logical form' will thus have to be defined in two (or more in case of a multilingual system) places. Note that this problem is a problem for all transfer systems; it is not restricted to our specific implementation of a transfer system (although the examples where the problem shows up may be specific to our grammars).
Some constructions such as control verbs and relative clauses may be represented using reentrancies; for example `the soldiers tried to shoot the president' may be represented by a feature structure where the first argument of `try' is reentrant with the first argument of `shoot', cf. Figure 13. The translation of such logical forms to Dutch equivalents can be defined as in rule 14. However, it is not clear that such reentrancies are always as local as in this case; if the reentrancies can be further away the transfer grammar will have to be complicated (eg. by a threading mechanism) to be able to translate such constructions, although a transfer writer would prefer the possibility to state that `reentrancies should be copied over' (cf. ).