'Constraint based on Boolean Decision Variable

Working on an assignment for my Masters program and am running into an issue with CPLEX

Without copy/pasting entire problem, the gist is an optimization problem regarding Farms and Plants with transportation and fixed costs etc.

One of the parts of the problem is Farm4 and Farm5 are a package deal (Either buy both, or buy neither) and I am not sure how to code that in CPLEX

Below is the code for the Farm Strings

{string} Farm = {"F1", "F2", "F3", "F4", "F5", "F6"};

And below is the code the the boolean decision variable to include each farm's fixed cost

dvar boolean Purchase[Farm];

I have tried a few things, but CPLEX doesn't allow me to do a constraint based upon a dvar.

Does anyone know anyway I can make this work?

Thanks in advance!

Solution 1:^[1]

Purchase["F4"]==Purchase["F5"];

Works fine for buy both or neither

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Solution	Source
Solution 1	Alex Fleischer