code: private static <K, V> Set<Entry<K, V>> constrainedEntrySet(
        Set<Entry<K, V>> entries, MapConstraint<? super K, ? super V> constraint) {
      return new ConstrainedEntrySet<K, V>(entries, constraint);
    }
docs: Returns a constrained view of the specified set of entries, using the
  specified constraint. The {@link Entry#setValue} operation will be verified
  with the constraint, along with add operations on the returned set. The
  {@code add} and {@code addAll} operations simply forward to the underlying
  set, which throws an {@link UnsupportedOperationException} per the map and
  multimap specification.
  
  <p>The returned multimap is not serializable.
  
  @param entries the entries to constrain
  @param constraint the constraint for the entries
  @return a constrained view of the specified entries