Special rules have had to be invented. A local rule in the UK was that if you were a member of a UK exchange and you traded one of its exchange-listed equities by any means, you had to report that trade to the exchange. The trade information would then become part of the exchange's outbound datafeed. A local rule in France (an example of the old "concentration rules" in many European countries) was that all trading in France of equities listed on a French exchange had to be carried out on-exchange.

Then there are the "incentive rules" - if an investment firm does a large "block" trade and reports it as being done under the rules of the exchange, it can delay publishing information about the trade for 5 days. This must be one of the weirdest rules - it claims that hiding major trading activity by one party from all of the rest of the market for five days is in the interest of the market.

And of course there has been the bending of rules.

It has been a battle between non-transparent trading in the interest of the client, transparent trading in the interest of the market, and policing and enforcement of the rules.

Dark Pools are basically the application of new technology to replace older trading methods - voice, telephone, down the wine bar, etc. Dark Pools don't change the problems - they just make the problems happen faster.