How to make production/attraction values zero?

Please help me to script such that negative trips aren't possible (due to a negative regression constant) and so that if the independent variable(s) is zero I don't get positive trips (due to a positive regression constant). what are the control statements and keywords to be used to do so? 

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  • For production/attraction equations, some zones have no value for independent variables associated with production/attraction equations. For some zones, independent variables values multiplied by coefficient result less than negative constants. Consequently, we will have negative trips in the output table. I want to script such that any values less than zero should equal zero, and; if the independent variable is zero, the number of trips has to be zero regardless of the positive constant in the regression equations.

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