A Goal-Oriented Algorithm for Unification in EL w.r.t. Cycle-Restricted TBoxes

These two concept descriptions are not equivalent, but they are nevertheless meant to represent the same concept. They can obviously be made equivalent by treating the concept names Head_injury and Severe_finding as variables, and substituting the first one by Injury u ∃finding_site.Head and the second one by ∃severity.Severe. In this case, we say that the descriptions are unifiable, and call the substitution that makes them equivalent a unifier. Intuitively, such a unifier proposes definitions for the concept names that are used as variables: in our example, we know that, if we define Head_injury as Injury u ∃finding_site.Head and Severe_finding as ∃severity.Severe, then the two concept descriptions (1) and (2) are equivalent w.r.t. these definitions. Here equivalence holds without additional GCIs. To motivate our interest in unification w.r.t. GCIs, assume that the second developer uses the description