Observe that abstraction is always well defined. The third condition captures the essence of the concept of abstraction.
The reader may wonder why we define abstractions only over one single multivalued property a. It could indeed be useful to group together objects that agree on a set of functional or multivalued properties. However, it can be shown that abstraction over functional properties is expressible using the other GOOD operations introduced in this section. Furthermore, abstraction
over multiple properties can always be reduced to abstraction over one single property. More details on the expressive power of abstraction are given in [32].