Bug 123972 [Undo] Support approval of undoable operation execution
In GMF we do some common validation on our commands before we proceed to execute, undo or redo them. This validation is external to the command's ability to execute, undo or redo itself. Registering an IOperationApprover with the operation history for this of validation would give us an elegant solution. However, the IOperationApprover only supports approval of undo and redo. We also require support for approval of operation execution. Would you consider adding a proceedExecuting() method to the IOperationApprover interface, or to an extension of the IOperationApprover interface?
bundles/org.eclipse.core.commands/src/org/eclipse/core/commands/operations/IOperationHistory.java
*            made to register an instance which is already registered with
	 *            this instance, this method has no effect.
	 * 
	 * @see org.eclipse.core.commands.operations.IOperationApprover
	 */
	void addOperationApprover(IOperationApprover approver);
	/**
	 * <p>
	 * Add the specified listener to the list of operation history listeners
	 * that are notified about changes in the history or operations that are
	 * executed, undone, or redone.
	 * </p>
	 * 
	 * @param listener
	 *            the IOperationHistoryListener to be added as a listener. Must
	 *            not be <code>null</code>. If an attempt is made to register
	 *            an instance which is already registered with this instance, this
	 *            method has no effect.
	 * 
	 * @see org.eclipse.core.commands.operations.IOperationHistoryListener
	 * @see org.eclipse.core.commands.operations.OperationHistoryEvent
	 */
	void addOperationHistoryListener(IOperationHistoryListener listener);
	/**
	 * <p>
	 * Close the current operation. If the operation has successfully completed,
	 * send listeners a <code>DONE</code>, <code>UNDONE</code>, or
	 * <code>REDONE</code> notification, depending on the mode. Otherwise send
	 * an <code>OPERATION_NOT_OK</code> notification. Add the operation to the
	 * history if specified and send an <code>OPERATION_ADDED</code>