docs: public void markUnresolvable(EObject referenced) {
  		if (!referenced.eIsProxy()) {
  			throw new IllegalArgumentException("Cannot mark an instance as unresolvable if it is already resolved");
  		}
  		URI proxyURI = ((InternalEObject)referenced).eProxyURI();
  		getUnresolvableURIFragments().add(proxyURI.fragment());
  	}