Erasing initial gains,the Sensex at the Bombay Stock Exchange was trading 13 points in the negative territory at 19,623 in afternoon deals, a short while ago. The 30-share index had opened over 106 points in the green, on sustained buying by funds and retail investors amid a firm trend in other Asian bourses.The Nifty at the National Stock Exchange was trading almost flat at 5,941 points.