A Computational Account of Modality-based Case Frame Transformation

Verb modality presents a major processing obstacle in any NLP application, and can be overcome either by exhaustively listing all possible modal variants of any given verb, or by deriving any modality-based valency alterations in situ. Given the combinatorial complexity of any exhaustive listing attempt, we adopt this second methodology in proposing a modular set of valency frame transformations to dynamically recreate the appropriate valency frame. Modularity in transformation produces the advantage that any permutation of modal transformation types can be modelled, while maintaining linguistic accountability within the rule sets. The drawback of rigidly adhering to a modular rule set is that any ambiguity of transformation for the modal types must be reproduced in the associated rule sets. This is facilitated by allowing any given rule set to output multiple valency frame derivations, and having multiple valency frame derivation candidates cascade through the rule sets in parallel. Final selection of the appropriate valency frame is then made by way of case slot correspondences with input, and selectional restrictions on each case slot. Selectional restrictions are indexed from the corresponding case slot in the original valency frame, and are considered to be unaffected by the derivation process, except where marked otherwise in the rule sets. The proposed case frame transformational rule sets are applied sequentially, according to the linear (left-toright) ordering of modal auxiliaries suffixing the verb stem. To take an example of the transformation process, the verb tabe-sase-rare-ru “to eat-cause-passpres” would involve the application of the causative, followed by the passive, rule sets. In the case of a causativised passive such as sika-rare-sase-ru “to scoldpass-cause-pass”, this ordering would be reversed. The principle mechanism called upon in implementing the transformational rule sets is grammatical roles, and hence some indication of the grammatical role of each case slot must be contained within the original valency frame. Within the rule sets, the only alterations made in transformation are to grammatical roles and associated case marking.