/** {@inheritDoc} */
 public boolean retainAll(TDoubleCollection collection) {
   if (this == collection) {
     return false;
   }
   boolean modified = false;
   TDoubleIterator iter = iterator();
   while (iter.hasNext()) {
     if (!collection.contains(iter.next())) {
       iter.remove();
       modified = true;
     }
   }
   return modified;
 }
 @Override
 public boolean contains(double o) {
   synchronized (mutex) {
     return c.contains(o);
   }
 }