About error using Java generics: "type parameter S is not within its bound"

I am writing some classes using Generics but I can't find a solution for the class SolutionsSubset and so I a getting the error "type parameter S is not within its bound". I have read previous questions about the same error but I can't solve it for my case. Could anybody help me to improve my knowledge about generics? Any reference to a good book (I can find in google a lot of information but if someone can reccommend a book, tutorial, etc. will be welcome). Although I tried to keep in mind the rules to ask a question but I apologize if my question doesn't fulfill these rules.

I have the following classes and interfaces:

public interface Subset<T extends Comparable<T>> extends Comparable<Subset<T>>
public class MathSubset<T extends Comparable<T>> extends TreeSet<T> implements Subset<T>

public interface Solution<T extends Comparable<T>>

public interface Solutions<S extends Solution<?>> extends Iterable<S>
public class SolutionsSubset<S extends Solution<?>> extends MathSubset<S> implements Solutions<S>

I need that Subset extends Comparable. In SolutionsSubset, the class MathSubset stores Solution objects. How do I have to change these definition to make it work?

Thanks you in advance