Constraining Predicted Values to be Greater Than 0

I have a WLS regression with 1 dependent variable and 3 independent variables.  I wish to constrain the predicted values (the fitted values) so that they are greater than zero (i.e. they are positive).  I do not know how to impose this constraint in R.  Please respond if you have any suggestions.

There are some previous postings about constraining the coefficients, but this won't accomplish what I am trying to do.  The coefficients can be negative, just as long as the predicted values are positive.

Thank you in advance for your time.

Westley A. Ritz
Analyst
215-641-2243
writz at trchome.com

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