Atom "Type Parameter Constraints" for the dedicated novice?

[waij]

Atom Type Parameter Constraints is a bit difficult for this dedicated novice. In Resilience.scala I have no idea where case class _Val() extends Val comes from. We did not cover that in the atom Enumeration. Why is there a difference, what does it mean? It goes downhill from there and I do not understand the principle of constraints from this code. I could not solve exercise 1 either.

I do understand Constraint.scala and could solve exercises 2 and 3.