PREDICTABLY, rugby league defector Sonny Bill Williams is threatening to hit the NRL with a writ to say that its salary cap is a restraint of trade. But experts say a salary cap — like the AFL's — is unlawful in the same way a nuclear bomb is a weapon: unarguably, but also improbable that it ever will be exploded. In nearly a quarter of a century, the AFL's button has not been pushed.

There are many reasons, but the best is also the most perverse. AFL is the biggest football code in Australia, but is not played elsewhere. For years, the other codes have mocked AFL as a big fish in a small pool.

Now, as the Williams case rocks the NRL, it can be seen that the AFL's isolation is its protection. No AFL player will be offered, as was Williams, more than three times as much to play in another country. There can be no restraint of trade because there is no trade, because there is no market.

Within Australia, there is. But it is, by consensus, regulated. All 16 AFL clubs agree to the salary cap, and so does the players' association, so for any one club to sue for restraint of trade would be to renege on a deal. One lawyer yesterday likened it to 16 men sitting around a table, each holding the others by the short and curlies, all daring one another to squeeze. It's a less elegant analogy than the nuclear deterrent, but it makes the point.

In any case, an action would be almost certain to fail. In 1986, Gary Buckenara, under contract to Hawthorn, wanted to join the newly formed West Coast Eagles, and sued the Hawks for restraint of trade. In 1988, Judge Crockett ruled that his Hawthorn contract was binding. "A court will be very slow to find a provision to be a restraint during the period of service required by the contract to be rendered," he said. Resigned, Buckenara played in Hawthorn premierships that year and the next.

Competition law says restraint of trade is prima facie illegal, but makes two exceptions: that it is reasonable in the circumstances, and that it is in the public interest. If challenged, the AFL would argue strenuously about public interest, saying that the salary cap and draft have made the competition what it is today — a competition — and that it is preferable by far than to have Collingwood, Carlton and Essendon play each other, ad nauseum. In this little pool, that is how it would be.

That is not to say the AFL is blithely unconcerned. The restraint-of-trade bomb, however benign, is still a bomb. In 1983, its previous system governing player movement was demolished as restraint of trade in court by Silvio Foschini, who refused to move with South Melbourne to Sydney. This led to the formulation of the draft and salary cap.

But there are, in the Williams case, other implications for the AFL, and for the wider sporting community. One is the enforceability of contracts; neither players nor clubs are much fussed about honouring them.

Just last year, Williams signed a five-year contract with the Bulldogs, pledging himself for life. Life turned out to be ephemeral, five years little more than one. The best the Bulldogs can hope for now is to sue him for damages, and the French club rugby Toulon for inducing him to break a contract. But both are far away, in another jurisdiction. And Williams is a New Zealander anyway.

Another issue is globalisation and how to deal with it. As the balance of power in cricket moved with the money to India, so in rugby has it moved from league to union, and from the southern hemisphere to the northern.

Australian soccer folk were laughing at this yesterday; they've been through this already, many years ago, and are out the other side. "Welcome to the real world, fellas, and it's not an easy place," wrote Michael Cockerill of the Sydney Morning Herald and Fox Sport. He predicted, as have others, a hastening of inevitable merger of league and union, effectively, a pooling of their resources.

The AFL is insulated from the forces of globalisation now, but for how long? Hitherto, it has been a self-contained game, with a discrete playing group; no elite player has moved between the AFL and another code.

Earlier this year, rugby union chief John O'Neill noted his code and AFL increasingly were recruiting for similar body types. At least two players with rugby league backgrounds are now playing senior AFL. The boundaries are blurring.

If and when the day comes when an AFL player is made an offer that — contract or no contract — he cannot refuse by another code in another place, can the AFL survive it?