Assert.isNotNull(group);
if (group == null) throw new IllegalArgumentException("group may not be null");
	public void addGroup(LinkedPositionGroup group) throws BadLocationException {		Assert.isNotNull(group);		if (fIsSealed)			throw new IllegalStateException("environment is already sealed"); //$NON-NLS-1$		if (fGroups.contains(group))			// nothing happens			return;		enforceNestability(group);		enforceDisjoint(group);		group.setEnvironment(this);		fGroups.add(group);	}