code: 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());
  	}
docs: Marks the given proxy as unresolvable. Further attempts to resolve it by means of {@link #getEObject(String)}
  will yield <code>null</code>.
  @since 2.4