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/IOperationHistory.java
* 
	 * @return the IStatus indicating whether the execution succeeded.
	 * 
	 * <p>
	 * The severity code in the returned status describes whether the operation
	 * succeeded and whether it was added to the history. <code>OK</code>
	 * severity indicates that the execute operation was successful and that the
	 * operation has been added to the history. Listeners will receive
	 * notifications about the operation's success (<code>DONE</code>) and
	 * about the operation being added to the history (<code>OPERATION_ADDED</code>).
	 * </p>
	 * <p>
	 * <code>CANCEL</code> severity indicates that the user cancelled the
	 * operation and that the operation was not added to the history.
	 * <code>ERROR</code> severity indicates that the operation did not
	 * successfully execute and that it was not added to the history. Any other
	 * severity code is not specifically interpreted by the history, and the
	 * operation will not be added to the history. For all severities other than
	 * <code>OK</code>, listeners will receive the
	 * <code>OPERATION_NOT_OK</code> notification instead of the
	 * <code>DONE</code> notification if the execution was approved and
	 * attempted.
	 * </p>
	 * 
	 * @throws ExecutionException
	 *             if an exception occurred during execution.
	 * 
	 */
	IStatus execute(IUndoableOperation operation, IProgressMonitor monitor,
			IAdaptable info) throws ExecutionException;