code: @VisibleForTesting
  	protected void assertValidExternalCounterStatus(final String counterName, final CounterStatus incomingCounterStatus)
  	{
  		Preconditions.checkNotNull(counterName);
  		Preconditions.checkNotNull(incomingCounterStatus);
  
  		if (incomingCounterStatus != CounterStatus.AVAILABLE && incomingCounterStatus != CounterStatus.READ_ONLY_COUNT)
  		{
  			final String msg = String.format(
  				"This Counter can only be put into the %s or %s status!  %s is not allowed.", CounterStatus.AVAILABLE,
  				CounterStatus.READ_ONLY_COUNT, incomingCounterStatus);
  			throw new CounterNotMutableException(counterName, msg);
  		}
  	}
docs: Helper method to determine if a counter can be put into the {@code incomingCounterStatus} by an external caller.
  Currently, external callers may only put a Counter into the AVAILABLE or READ_ONLY status.
  
  @param counterName The name of the counter.
  @param incomingCounterStatus The status of an incoming counter that is being updated by an external counter.
  @return