Subtype inheritance is closely patterned after the taxonomical principles of natural and mathematical sciences. A general rule of subtype inheritance is that the various heirs of a class represent sets of instances disjoint from each other. No closed figure, for example, is both a polygon and an ellipse. Multiple views Subtype inheritance is straightforward when a clear criterion exists to classify the variants of a certain notion. But sometimes several qualities vie for our attention.
Several strategies are available to address such conflicts.
They will be reviewed as part of the study of view inheritance later in this chapter. For features, the situation is a little more subtle. The subtype view implies that all operations applicable to an instance of the parent should be applicable to an instance of the heir. We may call such cases taxonomy exceptions . This would be unreasonable.