How to implement overriding function in protocol extension

I have a class that inherits from NSOperation and a protocol that the class should conform to:

protocol OperationProtocol { }

class Operation : NSOperation, OperationProtocol {}

I would like to make every operation have a main() method implemented in the Protocol which is called when the operation is run from a queue.

I tried this:

extension OperationProtocol where Self: NSOperation {
    func main() {
        doMyStuff()
    }
}

This main method isn't called. Putting "override" before it raises compiler error (method does not override method in superclass)

The only way I could make it, was naming the implementing function something else than "main()" and then override main() in the Operation and call the implementation from Protocol

class Operation : NSOperation, OperationProtocol {
    override func main() {
        self.elseThanMain()
    }
}

But I don't want to override main() in every subclass of Operation.

Is this even possible?

Thanks