STATA - Placing a Constraint on a Constant

I am running several iterations of a system of related equations. The first is four separate OLS equations estimating:

reg E_share lnp_cap lnp_lab lnp_energy lnp_mat

reg K_share lnp_cap lnp_lab lnp_energy lnp_mat

reg L_share lnp_cap lnp_lab lnp_energy lnp_mat

reg M_share lnp_cap lnp_lab lnp_energy lnp_mat

The second is a SUR regression of three of the four equations (the fourth is implicit):

sureg (eq1: E_share lnp_cap lnp_lab lnp_energy lnp_mat) (eq2: K_share lnp_cap lnp_lab lnp_energy lnp_mat) (eq3: L_share lnp_cap lnp_lab lnp_energy lnp_mat)

Finally, I would like to run a SUR regression as above, but with a constraint that the constants terms for all four equations sum to 1. I'm not sure what the syntax for this constraint is, as the variables I am imposing the constraint on are parameters and not actual variables in my data, and was wondering if someone has an idea of how to proceed. Thanks!