Hello, I'm using the following commands after which I would like to set up constraints for the parameters before opting to maximize ML. The restrictions are on the coefficients, all of which I want to be positive. Would somebody be kind enough to point me toward the right direction on how to setup the constraints?

global xvar x1 x2 G

xtset f t

sfpan y, dist(t) mu() prod frontier($xvar) usigmas vsigmas i(f) inv

quietly ml max, difficult nrtol(0.001) gtol(0.001)

scalar ll_inv = e(ll)

by f: egen smallT = min(t)

generate timeT = (t - smallT)

sfpan y, dist(t) mu() prod frontier($xvar) usigmas vsigmas gamma(timeT, nocons) i(f)

sf_srch, frontier($xvar) usigmas() vsigmas() n(1) gamma(year2) mu() fast