In order to try and uninstall a facet from a project I must check if it is an instance of MutableFaceted and then cast like so:

if (project instanceof MutableFaceted) {
  ((MutableFaceted<ProjectFacet>) project).uninstall(someFacet);
}

But as far as I can tell, every Project implementation implements MutableFaceted, so it would be nice if Project extended MutableFaceted so that I could avoid casting.

Alternatively, if there is a good reason for having Project implementations which are not MutableFaceted, perhaps there could be a MutableProject interface for convenience:

public interface MutableProject extends Project, MutableFaceted<ProjectFacet> {}