The generation algorithm defined above chooses particular lexical forms on-line. This approach can lead to a certain amount of unnecessary nondeterminism. The choice of a particular form depends on the available semantic and syntactic information. Sometimes there is not enough information available, to choose a form deterministically. For instance, the choice of verb form might depend on syntactic features of the verb's subject, available only after the subject has been generated. This nondeterminism can be eliminated by deferring lexical choice to a post-process. Inflectional and orthographical rules are only applied when the generation process is finished and all syntactic features are known. In short, the generator will yield a list of lexical items instead of a list of words. To this list the inflectional and orthographical rules are applied.
The MiMo2 system [103,104] incorporates such a mechanism into the previous generation algorithm quite successfully. Experiments with particular grammars of Dutch, Spanish, and English have shown that the delay mechanism results in a generator that is faster by a factor of two or three on short sentences. Of course, the same mechanism could be added to any of the other generation techniques discussed in this chapter; it is independent of the traversal order.
The particular approach to delaying lexical choice found in the MiMo2 system relies on the structure of the system's morphological component as presented in figure 3.12.
The figure shows how inflectional rules, orthographical rules, morphology and syntax are related: orthographical rules are applied to the results of inflectional rules. These inflectional rules are applied to the results of the morphological rules. The result of the orthographical part are then input for the syntax. However, in the lexical-delayed scheme, the inflectional and orthographical rules are delayed. During the generation process the results of the morphological grammar are used directly. It should be emphasized that this is possible, only because the inflectional and orthographical rules are monotonic, in the sense that they only further instantiate the feature structure of a lexical item, but do not change it. This implies, for example, that a rule that relates an active and a passive variant of a verb will not be an inflectional rule but rather a rule in the morphological grammar (as it changes for example the subcategorization requirements of the verb). The rule that builds a participle from a stem may in fact be an inflectional rule if it only instantiates the feature vform, for instance. When the generation process proper is finished the delayed rules are applied and the correct forms can be chosen deterministically.
The delay mechanism is useful in the two following general cases:
Firstly, and most importantly, the mechanism is useful if an inflectional variant depends on syntactic features not yet available. The particular choice of whether a verb has singular or plural inflection, depends on the syntactic agreement features of its subject; these may only be available after the subject has been generated. Other examples may include the particular choice of personal and relative pronouns, and so forth.
Secondly, delaying lexical choice is useful when there are several variants for some word, that are equally possible, because they are semantically and syntactically identical. For example, a word may have several spelling variants. If we delay orthography then the generation process computes with only one ``abstract'' variant. After the generation process is completed, several variants can be filled in for this abstract one. Examples from English include words that take both regular and irregular tense forms (e.g. ``burned/burnt''); and variants such as ``traveller/traveler,'' ``realize/realise,'' etc.