Generalize guice's robot-legs example with Multibinding

I have this use case that is very similar to the robot-legs example of Guice, except I don't know how many "legs" I have. Therefore I can't use the annotations needed for the robot-legs example.

I expect to gather all these "legs" in an java.util.Set with Guice's Multibindings extension.

Technically, in a PrivateModule I would like to expose an implementation directly as an element of the set that will be provided by the Multibindings extension. I just don't know how to do that.

For reference and code example, see the robot-legs example here: http://code.google.com/p/google-guice/wiki/FrequentlyAskedQuestions#How_do_I_build_two_similar_but_slightly_different_trees_of_objec

Here's my precise use-case:

I have the following:

// Main application
public interface MyTree {...}
public interface MyInterface {
  public MyTree getMyTree() {}
}
public abstract class MyModule extends PrivateModule {}
public class MyManager {
  @Inject MyManager (Set<MyInterface> interfaces){ this.interfaces = interfaces }
}
public class MainModule extends AbstractModule {
  public void configure () {
    // Install all MyModules using java.util.ServiceLoader.
  }
}


// In expansion "square.jar"
public class SquareTree implements MyTree {...}
public class SquareImplementation implements MyInterface {
  @Inject SquareImplementation (MyTree tree) { this.tree = tree; }
  public MyTree getMyTree () { return this.tree; }
}
public class SquareModule extends MyModule { // correctly defined as a ServiceLoader's service.
  public void configure () {
    // How to make this public IN a multibinder's set?
    bind(MyInterface.class).to(SquareImplementation.class);

    // Implementation specific to the Squareimplementation.
    bind(MyTree.class).to(SquareTree.class);
  }
}