st: Positivity

And I have another question; does anybody know how I could programme positivity for some coefficients in Stata?
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Follow-Ups:
- st: RE: Positivity
  - From: "Nick Cox" <[email protected]>
- st: RE: Positivity
  - From: "Rodrigo Alfaro A." <[email protected]>
- Re: st: Positivity
  - From: "Sergiy Radyakin" <[email protected]>

References:
- st: insufficient memory for matrix 11 r(950)
  - From: "Victor, Jennifer Nicoll" <[email protected]>
- st: RE: insufficient memory for matrix 11 r(950)
  - From: "Nick Cox" <[email protected]>