When China bulls tire of taking a product’s price, multiplying by 1.3bn and getting excited, they turn to the stock market. Imagine, they say, if Shanghai’s A-share market was open to outsiders. Multiply the earnings of companies with 1.3bn customers by a preferred price/earnings ratio, and you have the world’s biggest market. Fair enough in theory. But China is where theories go to die. Revived plans to link the Hong Kong and Shanghai exchanges will be a practical test.

Allowing up to Rmb23.5bn ($3.8bn) of daily cross-border trading in dual-listed stocks and selected blue-chips will sound familiar to anyone who remembers the 2007 attempt to form a similar “through train”. China had $1.3tn in foreign exchange reserves then, and was thought to be awash with cash that needed an outlet. Stocks in Shanghai were trading at an 80 per cent premium to their offshore counterparts. Seven years later, China’s reserves have tripled, China is feared to be awash in debt and Hong Kong stocks have the premium, of 6 per cent.

The authorities are not just sticking their toes in the waters of reform, as they often do. The numbers?are?big. The Rmb300bn total allowed into Shanghai would add a fifth to the current $216bn in quotas through various qualified foreign institutional investor (QFII) systems. In trading volumes, the Rmb13bn that can cross daily into mainland stocks is 14 per cent of the average value traded. The Rmb10.5bn heading to Hong Kong equates to a fifth of average trading there.