Is there any Trading systems model in stock or forex or commodities trading which contains a Portfolio of stocks or index or forex currencies and choose the right stock or currency for right time. i.e., Play only a single or two stock or currency at a given time choosing among the portfolio you choose and dynmically changing the portfolio depening on the market conditions. NORMALLY IN STOCK SYSTEM DESIGN, THEY FOLLOW ONLY A SINGLE STOCK OR CURRENCY . INSTEAD OF THAT METHOD, I WANT TO KNOW IS THERE ANY SYTEM OR METHOD AS I MENTIONED ABOVE WITH MULTIPLE STOCKS or currency etc., following but playing only one or 2 items only at a given time DEPENDING ON THE SITUATION of the markets. Is there any system avilable for sale in the market as I mentioned above?