[http://developer.apple.com/documentation/Cocoa/Reference/ApplicationKit/ObjC_classic/TypesAndConstants/AppKitTypes.html#//apple_ref/doc/uid/20000019/BAJJFEEE]

[http://developer.apple.com/documentation/Cocoa/Reference/ApplicationKit/ObjC_classic/Protocols/NSDraggingInfo.html]

here are the main drag operations:

� Control key: NSDragOperationLink

� Option key: NSDragOperationCopy?

� Command key: NSDragOperationGeneric