use cobbdouglas, clear

gen lcap=log(cap)
gen llab=log(lab)
gen lprod=log(prod)

regress lprod llab lcap
test llab+lcap = 1

*Two ways of imposing the constraint.
gen lpc=lprod-lcap
gen llc=llab-lcap
regress lpc llc

constraint 1 lcap+llab = 1
cnsreg  lprod llab lcap, constraints(1)