code: public static <K,V> Relation<K,V> unmodifiableRelation(final Relation<K,V> rel) {
  	return new UnmodifiableRelation<K,V>(rel);
      }
docs: Returns an unmodifiable wrapper backed by the given relation
  <code>rel</code>.  This allows "read-only" access, although
  changes in the backing collection show up in this view.
  Attempts to modify the relation will fail with {@link
  UnsupportedOperationException}.