Rules of inheritance hierarchies

Before looking into linearization rules, we need to be clear on some inheritance rules in Scala:

• In Java, even if a class does not explicitly extend another one, its superclass will be java.lang.Object. The same stands for Scala, and the equivalent base is AnyRef.
• There is a similarity between directly extending a trait and extending the trait superclass and mixing the trait in using the with keyword.

Using those rules, we can always get to a canonical form for all traits and classes, where the base class is specified using extends and then all traits are added using the with keyword.