code: @Nonnull
    public EChange add (@Nonnull final CALLBACKTYPE aCallback)
    {
      ValueEnforcer.notNull (aCallback, "Callback");
  
      return m_aRWLock.writeLocked ( () -> m_aCallbacks.addObject (aCallback));
    }
docs: Add a callback.
  
  @param aCallback
  May not be <code>null</code>.
  @return {@link EChange}