[Gretl-users] Coefficient restrictions

One of my former students has this problem: "The project involves regressing the returns from a portfolio against the returns from several market benchmarks (the S&P500, the S&P Midcap 400, the S&P Small-cap 600, and treasury bills). When the regression is restricted so that all of the coefficients are non-negative and sum to 1, the coefficients will identify which asset classes best explain the portfolio's returns, and therefore what style the portfolio manager is using. I figured out how to get the coefficients to sum to one using linear restrictions, but I can't seem to find any function or command in GRETL which will impose the non-negative restriction on the regression. Any ideas?" I don't know what to tell him. Suggestions? Thanks, Wilson Mixon