Inequations

Feature structures may also incorporate inequational constraints following (Carpenter 1992), which is in turn based on the notion of inequation in Prolog II (Colmerauer 1987). For instance, we might have the following representation of the semantics of a sentence:

  SEM binary_rel
      REL know
      ARG1  [0] referent
            GENDER masc
            PERS third
            NUM sing
      ARG2  [1] referent
            GENDER masc
            PERS third
            NUM sing
  [0] =\= [1]

Below the feature information, we have included the constraint that the value of the structure [0] is not identical to that of structure [1]. As a result, we cannot unify this structure with the following one:

  REL know
  ARG1 [2]
  ARG2 [2]

Any attempt to unify the structures [0] and [1] causes failure.