Bug 123316 [Undo] - API BEHAVIOR - should always place a successfully undone/redone op on the redo/undo history
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/operations/DefaultOperationHistory.java
/**
	 * a map of undo limits per context
	 */
	private Map limits = Collections.synchronizedMap(new HashMap());
	/**
	 * the list of {@link IOperationHistoryListener}s
	 */
	ListenerList listeners = new ListenerList(ListenerList.IDENTITY);
	/**
	 * the list of operations available for redo, LIFO
	 */
	private List redoList = Collections.synchronizedList(new ArrayList());
	/**
	 * the list of operations available for undo, LIFO
	 */
	private List undoList = Collections.synchronizedList(new ArrayList());
	/**
	 * a lock that is used to synchronize access between the undo and redo
	 * history
	 */
	final Object undoRedoHistoryLock = new Object();
	/**
	 * An operation that is "absorbing" all other operations while it is open.
	 * When this is not null, other operations added or executed are added to
	 * this composite.
	 * 
	 */
	private ICompositeOperation openComposite = null;
	/**
	 * a lock that is used to synchronize access to the open composite.