code: private PropStat addPropStat(PropStat propStat)
  	{
  		// create Map if necessary
  		Map<ElementDescriptor<?>, PropStat> propStatByProperty = mPropStatByProperty;
  		if (propStatByProperty == null)
  		{
  			propStatByProperty = mPropStatByProperty = new HashMap<ElementDescriptor<?>, PropStat>(16);
  		}
  
  		Set<ElementDescriptor<?>> properties = propStat.getPropertyDescriptors();
  		if (properties != null)
  		{
  			for (ElementDescriptor<?> property : properties)
  			{
  				propStatByProperty.put(property, propStat);
  			}
  		}
  
  		if (mPropStatByStatus == null)
  		{
  			mPropStatByStatus = new HashMap<Integer, PropStat>(6 /* the average case has no more than 2 PropStats per Response */);
  		}
  
  		return mPropStatByStatus.put(propStat.getStatusCode(), propStat);
  	}
docs: Add the given {@link PropStat}. If there already is a {@link PropStat} with the same status code it's replaced by the new one and returned.
  
  @param propStat
  The new {@link PropStat}.
  @return The old {@link PropStat} instance with the same status code or <code>null</code> if there is none.