Trimming its initial gains, the Sensex at the Bombay Stock Exchange was trading 18 points in the positive territory at 18,433 in afternoon trade, when reports last came in. The 30-share index had opened 118 points in the green today, on sustained buying by funds, amid a firm trend in other Asian bourses.The Nifty at the National Stock Exchange was trading 4 points in the negative zone at 5,555.