docs: public boolean putReference(K key, ServiceReference<V> reference) {
          if (key == null || reference == null)
              return false;
  
          ConcurrentServiceReferenceSet<V> csrs = elementMap.get(key);
          if (csrs == null) {
              // No set for this key: allocate a new set that shares a context reference.
              ConcurrentServiceReferenceSet<V> newSet = new ConcurrentServiceReferenceSet<V>(referenceName, contextRef);
  
              // Try to add the set to the map: the value returned will be non-null if something else beat us there.
              ConcurrentServiceReferenceSet<V> currentSet = elementMap.putIfAbsent(key, newSet);
              csrs = (currentSet == null) ? newSet : currentSet;
          }
  
          return csrs.addReference(reference);
      }