code: public final void add(GroupAddress a, boolean isUpdating)
  	{
  		if (getMainAddress().equals(a))
  			throw new KNXIllegalArgumentException("address equals datapoint main address");
  		if (isUpdating) {
  			if (!updating.contains(a))
  				updating.add(a);
  		}
  		else {
  			if (!invalidating.contains(a))
  				invalidating.add(a);
  		}
  	}
docs: Adds a group address to this datapoint to indicate that KNX messages with that
  address are allowed to alter the associated datapoint state (i.e. a state value
  related to this datapoint).
  <p>
  A group address can be marked as updating a state or invalidating a state. An
  address is added at most once to each category.
  
  @param a the KNX group address to add
  @param isUpdating <code>true</code> to mark the address as updating this
  datapoint state, <code>false</code> to mark it as state invalidating