They may argue that the use of price-customisation software by sellers amounts to tacit collusion to overcharge particular groups of buyers.

ECONOMIST: Online prices

In practice, the conditions for such tacit collusion are quite rare.

ECONOMIST: Economics focus

But it may take a few years for the newcomers to get a national presence in long-distance and the scope for tacit collusion between the big two may be worrying.

ECONOMIST: A high-wired act

No evidence of tacit collusion was found.

BBC: Competition Commission raps Big Four accountants

Constantly providing subscribers with fancy new handsets and tacit price collusion among the three operators the same stodgy ones that had kept internet rates so high were to blame.

ECONOMIST: Face value

For example, with a large number of financial intermediaries it seems unlikely that they could nudge themselves into tacit risk collusion because the incentive to defect is would be strong unless you were sure that enough firms had already tacitly colluded that they could trip the slaughter rule themselves.

FORBES: Inverse Moral Hazard: A Slaughter Rule For Finance

"Any tacit or formal collusion between the two powers to disrupt the process by punishing the Palestinian people could be counterproductive and have devastating consequences, " he says.

BBC: NEWS | Middle East | Carter: Don't punish Palestinians