code: protected Set<String> getUpdatedRolesFrom(Set<String> roles) {
          Set<String> updatedRoles = Sets.newHashSet(roles);
          updatedRoles.addAll(_rolesAdded);
          updatedRoles.removeAll(_rolesRemoved);
          return updatedRoles;
      }
docs: Helper method for subclasses which, given a set of roles, returns a new set of roles with all added and removed
  roles from this modification applied.