text: OWL classes are assumed to overlap, i.e., by default they are not disjoint. This is often useful 
  because in OWL, unlike in most object-oriented models, multiple inheritance is not discouraged 
  and can be a powerful tool to model data. If we want classes to be disjoint, we must explicitly 
  declare them to be so. It is often a good development strategy to start with classes that are not 
  disjoint and then make them disjoint once the model is more fully fleshed out as it is not always 
  obvious which classes are disjoint from the beginning. 